The following *nearly* worked for me (and yes, the 'sleep' command
*is* necessary, grumble grumble):

virt-builder fedora-21 \
    --install docker \
    --run-command '
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
    '

It eventually prints:
[...]
511136ea3c5a: Pulling fs layer
00a0c78eeb6d: Pulling fs layer
834629358fe2: Pulling fs layer
511136ea3c5a: Verifying Checksum
511136ea3c5a: Download complete
00a0c78eeb6d: Verifying Checksum
00a0c78eeb6d: Download complete
511136ea3c5a: Pull complete
00a0c78eeb6d: Pull complete

followed by an error, presumably because 'docker pull' exits
with the wrong error code, or doesn't print an error message.
You could add ||: after the docker pull command to ignore that.

Proxy settings from outside virt-builder are, however, not
passed through to docker, although as far as I can see that
is a docker problem, not a virt-builder problem.  Similar
to: https://github.com/tmatilai/vagrant-proxyconf/issues/97

Hrm. I ran that and here is what I got:

$ virt-builder fedora-21 \
>     --install docker \
>     --run-command '
>       docker -H unix:///var/run/docker.sock -d &
>       sleep 5
>       docker pull fedora:21
>     '

[   4.0] Downloading: http://libguestfs.org/download/builder/fedora-21.xz
[   5.0] Planning how to build this image
[   5.0] Uncompressing
[  17.0] Opening the new disk
[  40.0] Setting a random seed
[  40.0] Installing packages: docker
[ 102.0] Running: 
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
    
[ 621.0] Setting passwords
virt-builder: Setting random password of root to 7II5KPoA7uHByrt1
[ 622.0] Finishing off
virt-builder: error: libguestfs error: umount_all: umount: /sysroot: 
umount: /sysroot: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

If reporting bugs, run virt-builder with debugging enabled and include the 
complete output:

  virt-builder -v -x [...]



I didn't dig too deeply, but is this sysroot error caused by something that the daemon is still holding on to? What about the sleep 5, why is this necessary? Is there a clean solution, or a bug I can file with the docker people?

Cheers

Yup I suspect the docker daemon is holding /sysroot open.  Probably
adding `killall docker' as the last command could fix that.  Or
sending a "terminate the daemon" command if there is one.

The sleep 5 was necessary for me - it could be that docker doesn't
create the socket before forking into the background (which would be
a bug in docker if true).

I've created a docker issue to see if those folks can help suggest a solution or a fix, so that this works together more elegantly.

https://github.com/docker/docker/issues/11241

$ cat vb.sh
#!/bin/bash

virt-builder fedora-21 \
    --install docker \
    --run-command '
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
      killall docker
    '

$ ./vb.sh 
[   2.0] Downloading: http://libguestfs.org/download/builder/fedora-21.xz
[   2.0] Planning how to build this image
[   2.0] Uncompressing
[  14.0] Opening the new disk
[  29.0] Setting a random seed
[  29.0] Installing packages: docker
[  62.0] Running: 
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
      killall docker
    
ftp://ftp.cogeco.net/fedora/linux/releases/21/Everything/x86_64/os/repodata/4c0ea0d0ca8fd81fd3a96cacabfbcf9e02c33125670505fcf20aacefab48df02-primary.sqlite.xz: [Errno 12] Timeout on ftp://ftp.cogeco.net/fedora/linux/releases/21/Everything/x86_64/os/repodata/4c0ea0d0ca8fd81fd3a96cacabfbcf9e02c33125670505fcf20aacefab48df02-primary.sqlite.xz: (28, '')
Trying other mirror.
Resolving Dependencies
--> Running transaction check
---> Package docker-io.x86_64 0:1.5.0-1.fc21 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package           Arch           Version                 Repository       Size
================================================================================
Installing:
 docker-io         x86_64         1.5.0-1.fc21            updates         5.8 M

Transaction Summary
================================================================================
Install  1 Package

Total download size: 5.8 M
Installed size: 26 M
Downloading packages:
warning: /var/cache/yum/x86_64/21/updates/packages/docker-io-1.5.0-1.fc21.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID 95a43f54: NOKEY
Public key for docker-io-1.5.0-1.fc21.x86_64.rpm is not installed
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-21-x86_64
Importing GPG key 0x95A43F54:
 Userid     : "Fedora (21) <fedora>"
 Fingerprint: 6596 b8fb abda 5227 a9c5 b59e 89ad 4e87 95a4 3f54
 Package    : fedora-repos-21-2.noarch (@anaconda)
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-21-x86_64
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : docker-io-1.5.0-1.fc21.x86_64                                1/1 
  Verifying  : docker-io-1.5.0-1.fc21.x86_64                                1/1 

Installed:
  docker-io.x86_64 0:1.5.0-1.fc21                                               

Complete!
time="2015-03-08T16:34:31-04:00" level="info" msg="+job serveapi(unix:///var/run/docker.sock)" 
time="2015-03-08T16:34:31-04:00" level="info" msg="Listening for HTTP on unix (/var/run/docker.sock)" 
time="2015-03-08T16:34:31-04:00" level="warning" msg="WARNING: Udev sync is not supported. This will lead to unexpected behavior, data loss and errors" 
time="2015-03-08T16:34:32-04:00" level="info" msg="+job init_networkdriver()" 
time="2015-03-08T16:34:32-04:00" level="info" msg="-job init_networkdriver() = OK (0)" 
time="2015-03-08T16:34:32-04:00" level="info" msg="WARNING: mountpoint for memory not found\n" 
time="2015-03-08T16:34:32-04:00" level="info" msg="Loading containers: start." 

time="2015-03-08T16:34:32-04:00" level="info" msg="Loading containers: done." 
time="2015-03-08T16:34:32-04:00" level="info" msg="docker daemon: 1.5.0 a8a31ef/1.5.0; execdriver: native-0.2; graphdriver: devicemapper" 
time="2015-03-08T16:34:32-04:00" level="info" msg="+job acceptconnections()" 
time="2015-03-08T16:34:32-04:00" level="info" msg="-job acceptconnections() = OK (0)" 
time="2015-03-08T16:34:36-04:00" level="info" msg="POST /v1.17/images/create?fromImage=fedora%3A21" 
time="2015-03-08T16:34:36-04:00" level="info" msg="+job pull(fedora, 21)" 
time="2015-03-08T16:34:36-04:00" level="info" msg="+job resolve_repository(fedora)" 
time="2015-03-08T16:34:36-04:00" level="info" msg="-job resolve_repository(fedora) = OK (0)" 
time="2015-03-08T16:34:36-04:00" level="info" msg="+job trust_update_base()" 
time="2015-03-08T16:34:36-04:00" level="info" msg="-job trust_update_base() = OK (0)" 
time="2015-03-08T16:34:37-04:00" level="info" msg="+job trust_key_check(/library/fedora)" 
time="2015-03-08T16:34:37-04:00" level="info" msg="-job trust_key_check(/library/fedora) = OK (0)" 
time="2015-03-08T16:34:37-04:00" level="info" msg="Image manifest for fedora:21 has been verified" 
511136ea3c5a: Pulling fs layer
00a0c78eeb6d: Pulling fs layer
834629358fe2: Pulling fs layer
511136ea3c5a: Verifying Checksum
511136ea3c5a: Download complete
00a0c78eeb6d: Verifying Checksum
00a0c78eeb6d: Download complete
511136ea3c5a: Pull complete
00a0c78eeb6d: Pull complete
834629358fe2: Verifying Checksum
834629358fe2: Download complete
834629358fe2: Pull complete
fedora:21: The image you are pulling has been verified. Important: image verification is a tech preview feature and should not be relied on to provide security.
Status: Downloaded newer image for fedora:21
time="2015-03-08T16:38:48-04:00" level="info" msg="+job log(pull, fedora:21, )" 
time="2015-03-08T16:38:48-04:00" level="info" msg="-job log(pull, fedora:21, ) = OK (0)" 
time="2015-03-08T16:38:48-04:00" level="info" msg="-job pull(fedora, 21) = OK (0)" 
/bin/sh: line 7: killall: command not found
virt-builder: error: 
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
      killall docker
    : command exited with an error

If reporting bugs, run virt-builder with debugging enabled and include the 
complete output:

  virt-builder -v -x [...]

$

doh :P this is why hacking on these things takes so long... it did seem to work though. Once more with feeling (and killall):

$ cat vb.sh 
#!/bin/bash

virt-builder fedora-21 \
    --install docker,psmisc \
    --run-command '
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
      killall docker
    '

$ ./vb.sh 
[   2.0] Downloading: http://libguestfs.org/download/builder/fedora-21.xz
[   2.0] Planning how to build this image
[   2.0] Uncompressing
[  14.0] Opening the new disk
[  26.0] Setting a random seed
[  26.0] Installing packages: docker psmisc
[  60.0] Running: 
      docker -H unix:///var/run/docker.sock -d &
      sleep 5
      docker pull fedora:21
      killall docker
    
[ 316.0] Setting passwords
virt-builder: Setting random password of root to hunter2
[ 316.0] Finishing off
                   Output file: fedora-21.img
                   Output size: 4.0G
                 Output format: raw
            Total usable space: 5.2G
                    Free space: 3.8G (72%)

Which seemed to work :)
So I might have a workaround, but there are definitely hacks involved. I see a few solutions:

1) Hopefully the docker people fix this daemon madness
2) If #1 is really impossible for some reason, perhaps virt-builder grows a --docker native command? The problem with this approach is that it might also want a rocket command, and others too. So perhaps something more general would be useful.

Yeah I suspect hard-coding docker daemons into virt-builder isn't
going to work upstream.  However suggestions for some kind of
generic --daemon or --service option are welcome ...

In general virt-builder's --run-command isn't that similar to
the guest environment.  Some things may be better done in
--firstboot-command.  See also:
http://libguestfs.org/guestfs.3.html#running-commands

I've patched vagrant-builder to use this workaround:

https://github.com/purpleidea/vagrant-builder/tree/feat/new-docker

Two notes:

1) I added the --selinux-enabled flag
2) I've wholly not tested this at all (not even once)

Figured I'd put this out here early. More permanent fixes either here and/or in docker are welcome, but probably not urgent at this time.

Cheers,
James

FWIW, I've pushed this to git master, and it seems to work :)

w00t! Thanks Rich.

I'd leave this open because it's kind of a hack, but if you prefer we close, that's fine too.

Cheers!

This product has been discontinued or is no longer tracked in Red Hat Bugzilla.

Reopening because Virtualization Tools has not been discontinued.