Put all install steps in the Dockerfile

Note: MySQL initialization still needs to be broken out to a first-run
process.
Split DB init into firstrun.sh

Also, pre-built configuration can be performed by editing the contents
of the config folder.
Fixed GitLab shell version
Moved mysql data to /srv/gitlab/mysql on firstrun
Statically set db username instead of using sed
All data is now moved into /srv/gitlab/data

This should make it possible for users to update GitLab by
rebuilding/pulling the image, replaceing the second half of firstrun.sh
with their DB migrations, and doing a docker run with the -v param
mounting the external folder containing the data to /srv/gitlab
internally.
Removed symlink for repo dir

It appears that GitLab doesn't like symlinks for some things, so I
edited some config files instead.
Copy config files instead of moving them
Wrote a much more detaild readme
Regenerate SSH host key on first run
Now using multline RUNs in the Dockerfile
Fixed typos and spelling