docs: cloudinit might fail if you bring your own Docker image

[bschaatsbergen]

Cloudinit performs a chown with the uid 100 (atlantis user) on the GCE Persistent Disk mount path - we should document this so that users are aware that when handrolling their own Docker image.

[bschaatsbergen]

This needs to be tested with a Docker image that's not using Atlantis as a base (as the base image sets the atlantis user to uid 100), but just installs the binary.

[nitrocode]

Exposing the script would be a good escape hatch for people using non standard docker images

[bschaatsbergen]

This is addressed by @d-costa in #131