Support host mounts in containers

[kegsay]

Developers often want to run new tests against a local version of their HS. This is cumbersome at present as you have to rebuild a docker image before you can run the test. Instead, we can expose directory on the container which will be volume mounted from a new env var e.g:

COMPLEMENT_MOUNT=/Users/me/my/local/dendrite
COMPLEMENT_BASE_IMAGE=dendrite-which-reads-mounted-dir:latest
go test ./...

• Should we just expose the source location or also the destination location e.g COMPLEMENT_MOUNT=/Users/me/my/local/dendrite:/data
• We should standardise the mounted directories (as /ca is done for COMPLEMENT_CA=1) as we will inevitably expand this in the future (e.g appservices need registration YAML files in the container)

We can either be prescriptive and say that we map to a static directory layout:

/complement
    |__ /appservice
    |__ /ca
    |__ /mount

or we can make this configurable e.g:

COMPLEMENT_MOUNT=/Users/me/my/local/dendrite:/mnt
COMPLEMENT_CA=/Users/me/my/ca:/certs
COMPLEMENT_AS=/Users/me/my/appservices:/as

This feels wrong though as it then ties env vars to a specific docker image which isn't what we're trying to do here (we want them to be interchangeable), so let's be prescriptive.

[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)

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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.