Toying with the build files in faasm, faabric, and cpp, I have noticed that there is a naming inconsistency in the tasks we use to manage git/code versions and the tasks to build docker images. These tasks do the same thing in all the repos, and we use them a lot. Ideally we'd have a CLI tool versioned in a different repo, but thats matter for another discussion.

In short, tasks to manage git are sometimes called inv release.* and other times inv git.*. Tasks to manage docker are sometimes called inv docker.* and other times inv container.*. In my opinion, inv git and inv docker are the most intuitive.

This is the first of a three-part PR that implements the following changes in faasm, faabric, and cpp:

1. Re-factor (if exists) inv release.* to inv git.*.
2. Add task to bump code version (if it does not exist) inv git.bump
3. Re-factor (if exists) inv container.* to inv docker.*
4. Make a common signature for inv docker.build. Stick to inv docker.build -c <ctr_name> [-c <ctr_name_2>] [--nocache] [--push]
5. Extend the use of venv-bm from faasm to all other repos (a bit unrelated but worth the effort).
6. In the process of doing 5, try (whenever reasonable) to reduce the number of layers (i.e. RUN commands) in our docker build files. This is recommended in docker's best practices. As always with style guidelines that we can't enforce, it's a bit of a best-effort thing.
7. Make THIS_DIR resolution zsh-proof (see Make scripts zsh proof faasm#666).

I am aware that 5 and 6 are a bit out of the scope, but they are small re-factors that should come with some performance/space benefits. That being said, I'd be happy to put them in different PRs.