Homogenise git and docker interface

[csegarragonz]

This is the third 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 #666).

Closes #666
For more background, see faasm/faabric#272.