Support custom bind-mounts

[jacob-hughes]

This adds a TRYCI_BINDMOUNTS environment variable, which can be used to mount arbitrary directories from the host to the docker container to be used in CI.

A common pattern I find myself myself repeating is using tryci to run the full test suite on yk and repeating it a lot once I inevitably find my change has broken something.

If my change includes an upstream change to ykllvm which hasn't landed yet, I can't take advantage of caching and have to wait almost an hour in-between builds.

This hack lets me reduce my tryci builds back down to about 10 minutes by letting me bind-mount a pre-built ykllvm from my host machine. It has similar uses for other submodules like bdwgc in alloy.

[ltratt]

Dumb question: should this be a flag to the tool or an environment variable?

[jacob-hughes]

I made it an environment variable so I could add the bind-mounts to my local machine once in my .zshenv (I use a different directory to /opt/ykllvm_cache on my desktop), but I don't have a particularly strong opinion about it.

[ltratt]

I'm generally averse to environment variables, as they're so easy to get wrong. If we can flip it to a CLI switch, I think that would be much less prone to error.

[jacob-hughes]

Can I force-push a fix for this that also rebases from master?

[ltratt]

If you force push and rebase at the same time, reviewing has to be started from scratch, so ideally "do one or the other".

[jacob-hughes]

Unfortunately while the core idea is the same, the code has changed enough that it probably needs re-reviewing. I can close it and re-raise a new one?

[ltratt]

OK, then please force push and rebase.

[jacob-hughes]

Force pushed and rebased.