Description of problem:

When you have your /etc/containers/registries.conf file in the 'old' YAML format, the registries.service gives a warning you should convert it to TOML:

Dec 04 15:18:13 coco.esat.kuleuven.be registries.log_warning[751]: /etc/containers/registries.conf is in YAML format and should be in TOML format. Back this file up and then use /usr/libexec/registries_migrator > /etc/containers/registries.conf to convert it to TOML.

However:

*) When you follow the recipe, it will simply wipe out your
   file (replaced with an essentially empty file).  Probably 
   the redirection problem (start outputting while still reading 
   the same file). Tool should read first the complete input file, 
   then spit out the result.
   
*) When you circumvent this problem by redirecting somewhere else, 
   it still fails to produce a correct file, as it puts quotes in
   the wrong place.

Version-Release number of selected component (if applicable):

F27: atomic-registries-1.20.1-3.fc27.x86_64
F26: atomic-registries-1.20.1-1.fc26.x86_64

How reproducible:

Always

Steps to Reproduce:
1. Check original correct YAML file: (I removed the comment)
 
   >> cat /etc/containers/registries.conf
   registries:
   - docker-registry.esat.kuleuven.be:5000
   - registry.fedoraproject.org
   - registry.access.redhat.com

2. Follow the recipe literally:

   >> /usr/libexec/registries_migrator > /etc/containers/registries.conf
 
   See the file gone:

   >> cat /etc/containers/registries.conf
   ["registries.search"]
   registries = []

   ["registries.insecure"]
   registries = []

   ["registries.block"]
   registries = []

3. Follow the recipe, but redirect elsewhere

   (After putting back the original obviously)
 
   >> /usr/libexec/registries_migrator > /etc/containers/registries.conf.new
   
   >> cat /etc/containers/registries.conf.new
   ["registries.search"]
   registries = ["docker-registry.esat.kuleuven.be:5000",    "registry.fedoraproject.org", "registry.access.redhat.com"]

   ["registries.insecure"]
   registries = []

   ["registries.block"]
   registries = []

   No it's not empty, but ... the quotes inside the []'s are afaik not
   valid TOML.

4. Try start registries service

   >> mv /etc/containers/registries.conf.new /etc/containers/registries.conf

   >> systemctl start registries.service
   Job for registries.service failed because the control process exited with error code.
   See "systemctl  status registries.service" and "journalctl  -xe" for details.

   Error is:
   Dec 07 13:39:11 coco.esat.kuleuven.be registries[11347]: Unable to load and parse /etc/containers/registries.conf

5. Remove the incorrect quotes from the file, so that it looks like;

   [registries.search]
   registries = ["docker-registry.esat.kuleuven.be:5000", "registry.fedoraproject.org", "registry.access.redhat.com"]

   [registries.insecure]
   registries = []

   [registries.block]
   registries = []
   
Additional info:

Not really convinced the the move from YAML to TOML was really worth the trouble. Keeping in mind that the only
thing it does, is output a single bash line for an environment variable ...