Allow to clone an external repo before the build

Nobody has claimed this yet.

In order to use code from a different repo we need to clone the repo from inside the docker image.

There are two main limitations of this approach as it's currently done:
- the tag/branch is hardcoded in the dockerfile (not configurable)
- if the repo is updated, the changes are not taken due to the docker cache, so we need to add ugly commands to override the cache such as `RUN echo "cache 2021-11-24"`

One nice way would be to add a new kind of dependency:

```yaml
harness:
dependencies:
hard:
soft:
build:
git:
- url: https://github.com/MetaCell/nwb-explorer.git
branch_tag: master
path: /git
```

Start by tracing how cloud-harness currently clones external repositories inside the Docker build and how the YAML dependency sections are parsed. Compare that flow with the proposed git dependency fields, then verify that branch or tag selection, destination path, and repository updates are configurable and no longer rely on manual cache-busting commands.

Written by the indexing model from the issue text.