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

[dustymabe]

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

[dustymabe]

Everything seems to be working for me now:

[core@coreos ~]$ id
uid=1000(core) gid=1000(core) groups=1000(core),4(adm),10(wheel),16(sudo),190(systemd-journal) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[core@coreos ~]$ docker ps -a
Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Get http://%2Fvar%2Frun%2Fdocker.sock/v1.38/containers/json?all=1: dial unix /var/run/docker.sock: connect: permission denied
[core@coreos ~]$ grep docker /etc/group
docker:x:982:
[core@coreos ~]$ sudo usermod -aG docker core
[core@coreos ~]$ grep docker /etc/group
docker:x:982:core
[core@coreos ~]$ exit
logout

Fedora 30.20190919.dev.2 (CoreOS preview)
Kernel 5.2.15-200.fc30.x86_64 on an x86_64 (ttyS0)

SSH host key: SHA256:/XxckNR4lI0OopENws0elbI/FviQ++IsXZY8a4W54Ow (ECDSA)
SSH host key: SHA256:iv4AfbeevHg9uwYvFaXIjqwy2PFTBdNS2jYwIo4ewxU (ED25519)
SSH host key: SHA256:s2YbJCdPvMIFsZEX4RVTG7nCtcj8TNF1bx5PUVUodos (RSA)
eth0: 10.0.2.15 fec0::5054:ff:fe12:3456
coreos login: core (automatic login)

Last login: Thu Sep 19 21:09:47 on ttyS0
ICMP traffic (ping) does not work with QEMU and user mode networking.
To exit, press Ctrl-A and then X.

[core@coreos ~]$ id
uid=1000(core) gid=1000(core) groups=1000(core),4(adm),10(wheel),16(sudo),190(systemd-journal),982(docker) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[core@coreos ~]$ docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

[miabbott]

LGTM