fedora-coreos-base: add the docker group to /etc/group file

This is a short term solution to fix an issue where running
`usermod -aG docker username` doesn't work.

Fixes: coreos/fedora-coreos-tracker#2

fedora-coreos-base.yaml

@@ -30,6 +30,11 @@ etc-group-members:
   - sudo
   - systemd-journal
   - adm
+  # Add the docker group to /etc/group
+  # https://github.com/coreos/fedora-coreos-tracker/issues/2
+  # This will be no longer needed when systemd-sysusers has been implemented:
+  # https://github.com/projectatomic/rpm-ostree/issues/49
+  - docker
 check-passwd:
   type: "file"
   filename: "passwd"