Description of problem:
.dockerignore `/bla` entry should ignore directory named "bla" in root, but it doesn't. The latest upstream docker release is not affected, so this bug is purely in the Fedora version of docker. Also see here for a documentation how .dockerignore should behave: https://docs.docker.com/engine/reference/builder/#dockerignore-file and here for an examination of upstream behavior: https://github.com/moby/moby/issues/36660#issuecomment-375143143

Version-Release number of selected component (if applicable):
docker-1.13.1-44.git584d391.fc27

How reproducible:
100%

Steps to Reproduce:
1. Run this script (provided by @thaJeztah of docker):
mkdir repro-36660 && cd repro-36660
mkdir bla
echo '/bla' > .dockerignore
cat > Dockerfile -<<EOF
FROM busybox
RUN mkdir /copied/
COPY . /copied/
RUN ls -la /copied/
EOF
docker build --no-cache .

Actual results:
Sending build context to Docker daemon 3.584 kB
Step 1/4 : FROM busybox
Trying to pull repository docker.io/library/busybox ... 
sha256:2107a35b58593c58ec5f4e8f2c4a70d195321078aebfadfbfb223a2ff4a4ed21: Pulling from docker.io/library/busybox
d070b8ef96fc: Pull complete 
Digest: sha256:2107a35b58593c58ec5f4e8f2c4a70d195321078aebfadfbfb223a2ff4a4ed21
Status: Downloaded newer image for docker.io/busybox:latest
 ---> f6e427c148a7
Step 2/4 : RUN mkdir /copied/
 ---> Running in 14322ef9b15e
 ---> f6b034673f5b
Removing intermediate container 14322ef9b15e
Step 3/4 : COPY . /copied/
 ---> 8cfc08ced82a
Removing intermediate container fa71cd3139a9
Step 4/4 : RUN ls -la /copied/
 ---> Running in d030a3756e15
total 8
drwxr-xr-x    3 root     root            56 Mar 22 07:18 .
drwxr-xr-x   14 root     root           162 Mar 22 07:18 ..
-rw-rw-r--    1 root     root             5 Mar 22 07:17 .dockerignore
-rw-rw-r--    1 root     root            68 Mar 22 07:17 Dockerfile
drwxrwxr-x    2 root     root             6 Mar 22 07:17 bla
 ---> 77d54bee1720
Removing intermediate container d030a3756e15
Successfully built 77d54bee1720
[jonas@falcon repro-36660]$

Expected results:
Sending build context to Docker daemon 3.584 kB
Step 1/4 : FROM busybox
Trying to pull repository docker.io/library/busybox ... 
sha256:2107a35b58593c58ec5f4e8f2c4a70d195321078aebfadfbfb223a2ff4a4ed21: Pulling from docker.io/library/busybox
d070b8ef96fc: Pull complete 
Digest: sha256:2107a35b58593c58ec5f4e8f2c4a70d195321078aebfadfbfb223a2ff4a4ed21
Status: Downloaded newer image for docker.io/busybox:latest
 ---> f6e427c148a7
Step 2/4 : RUN mkdir /copied/
 ---> Running in 14322ef9b15e
 ---> f6b034673f5b
Removing intermediate container 14322ef9b15e
Step 3/4 : COPY . /copied/
 ---> 8cfc08ced82a
Removing intermediate container fa71cd3139a9
Step 4/4 : RUN ls -la /copied/
 ---> Running in d030a3756e15
total 8
drwxr-xr-x    3 root     root            56 Mar 22 07:18 .
drwxr-xr-x   14 root     root           162 Mar 22 07:18 ..
-rw-rw-r--    1 root     root             5 Mar 22 07:17 .dockerignore
-rw-rw-r--    1 root     root            68 Mar 22 07:17 Dockerfile
 ---> 77d54bee1720
Removing intermediate container d030a3756e15
Successfully built 77d54bee1720
[jonas@falcon repro-36660]$