set "ENV container docker" for CentOS and friends

[mmckinst]

I was having similar problems to issue #109 where I'm using kitchen-docker to do acceptance testing for puppet modules on centos 7 with systemd. EG after running a puppet module, run serverspec or bats to ensure the module actually installed, configured, and started the service correctly.

With this patch and the below config, you can get systemd running on the centos7 docker container.

driver:
  name: docker
  privileged: true
  provision_command: 
    - /usr/bin/systemctl enable sshd
  run_command: /usr/sbin/init

https://developerblog.redhat.com/2014/05/05/running-systemd-within-docker-container/ and https://hub.docker.com/_/centos/ have a bunch of other things you might have to do regarding /sys/fs/cgroup and deleting some systemd "wants" files but I've found I don't have to do that. Someone else's mileage may vary.

[acondrat]

@mmckinst Thanks for the fix. I had a similar problem. My centos7 docker container was running fine on an Ubuntu host but not on a Centos host. The container is being started but it's missing sshd.

Would love to see it merged into master and released!

[trinitronx]

@mmckinst: This is excellent! Just having it in the main centos platform commands is great & the single environment variable shouldn't interfere with anything unless systemd is activated with the .kitchen.yml settings you mention! I've run into this magic incantation to get systemd working in Docker before, but surprisingly it wasn't well documented then. Yet, indeed as you've found, it's now documented by RedHat and the official CentOS Docker Hub image, which is also great!

Trying to band-aid patch this myself manually just to set an ENV variable felt wrong & easy to forget, and seeing that all the other pull requests for setting environment variables failed (#165, #123). Great to see this merged in!

[cruwe]

With recent debian:stretch containers, it is necessary to set container=docker as well. It might be an idea to reopen #192 and extend the fix similarly to debian and ubuntu. I have tested with ubuntu and it does not seem to hurt.