docker: add support for try-like repository (bug 1986575)

[shtrom]

As Try is a rather large repo, it's not always desirable to use it. Instead, the feature is set behind a docker compose profile which is not enabled by default, but can be on demand.

[zzzeid]

In theory, we could just create a small repo clone (similar to test-repo, first-repo, etc.) that allows multiple heads, and treat it as a try repo. That way it would be compatible with the existing repos. Thoughts?

[shtrom]

In theory, we could just create a small repo clone (similar to test-repo, first-repo, etc.) that allows multiple heads, and treat it as a try repo. That way it would be compatible with the existing repos. Thoughts?

One problem I ran into is that, to test the Try functionality, I needed something with mach. I also used the environment as a way to test changes to mach try, but that essentialy meant bringing an actual Firefox clone in, working from it, and pushing to a matching try repo.

So this is to support this use-case.