Support git auto tagging in publish command

[mefellows]

The git tagging feature won't work unless both git is installed and the volumes are mounted correctly.

I think we need to add this dependency to the image:

RUN apk add git

and demonstrate in the README how to add a volume mapping for .git into the container so that the repository is available.

Raised on slack forum this morning:

[bethesque]

I've added git. The docs already do say to mount the current volume -v ${PWD}:${PWD} so it should just work.

[bethesque]

Ah, right, the gem paths aren't right unless you're at $HOME. I've updated the image so that the CLI can be executed from any directory. And the docs.