Improve archive upload/download integration

Build upon the work done in #20

I think it would be a good idea to move the code out of `docker.py` and into its own file, as it's not very docker related (perhaps `archive.py`).

We should also use envvars instead of the dict defined in the source file. We can then uncomment the function calls in `commands.py` and put in a statement to exit early out of the `docker.upload_archives` and `docker.download_archives` functions if the AWS envvars aren't defined.

In `commands.py`, I think it makes sense to place `docker.upload_archives` under `build` and `docker.download_archives` under `start`. We would then also have to modify the *start* process to unpack the archive and use it, instead of building from scratch (this is a slightly bigger task than the above points).