Support host mounts in containers

[matrix-org]

Support host mounts in containers

[ShadowJonathan]

I think these mounts should be read-only, considering that Complement isn't supposed to do more than just read their code.

(I can think of write-specific side-effects in programming languages like python, which add write bytecode alongside the source code, or create directories like __pycache__, best to isolate that)

Any write-specific issues and behaviour can be fixed by layering a copy-on-write layer on top of it (though i can't remember exactly how or if thats possible with docker)

---

Regarding mapping; i think that complement should document well-known locations (/mnt, /certs, and /as in that example) within its container, and then have the developer submit a ;-seperated mount list to the runner.

[kegsay]

Thought about this a bit more and we need to allow arbitrary mounts so devs are free to mount things like GOPATH, pycache, etc to speed up edit-compile-run cycles... it also needs to be repeatable and not pollute the mounted volumes else we risk contaminating blueprints.