Add ENV var to control DEV workdir mount to host

[minottic]

using unison the bbackup folder on the host and the workdir in the container are synchronized (both ways)

[rkweehinzmann]

looks good.

[sbliven]

I'd like to discuss this more. Duplicating the data storage doesn't seem like the most efficient solution when we have mounts available. And unison can use a significant amount of resources for big changes like git checkouts.

[minottic]

sure, I think in person will be a little easier. High-level, the reason is: if you use mount points, it is hard to sync subsequent docker compose up -d with different ENVs, as different ENVs will need to write to different container dirs not to conflict. But then I think it would be confusing to have the changes separate, and so we would need to find a way to merge them. I implemented an alternative solution here: #384 but it had this problem. If we revise it slightly we could maybe use that instead of unison. I liked unison as it also does a backup, not only a local dev (and makes sure to keep everything in sync)