Support path to Dockerfile in Cross.toml

[gnzlbg]

Currently one can only provide an image:tag in the Cross.toml to use other docker images, which means that one has to build the images first, which can be a pain to do in CI.

It would be great if one could provide a path to a Dockerfile instead (or the docker context directory), so that cross would build this image for you, and use it.

[jamesmunns]

I'm happy to help point the way for someone if they would like to tackle this. I think this should be reasonable to implement for someone familiar with Docker.

[Dylan-DPC-zz]

@gnzlbg can you go ahead and submit a PR? we will help you out if needed. If not possible I'll try and do it.

[gnzlbg]

Does the following make sense?

• 1. add a context method to Toml https://github.com/japaric/cross/blob/master/src/main.rs#L470 that is similar to image but reads a context = path/to/folder where folder contains a Dockerfile. Update image and context to check that only one of both values is present, that is, if you use image = ... you can't use context = ... and vice-versa.
• 2. add an or_else call here in case image() returns none that calls context. If there is a context path, then just docker build ... the path, giving it a generic image name (e.g. cross-image-${TARGET}:0.1 or similar), and returning the generic image name

And... that would be it? Does it make sense to "clean up" the image on exit? Maybe the generic image name should have the path of the current directory hard-coded or something?

[gnzlbg]

I've sent a PR, but I don't know how cross is tested, nor how to add tests for this. I would expect that, since cross is a CLI app, that it would be using something like assert_cli to test its behavior, but it seems that's not the case.

[Alexhuszagh]

This is done now that pre-build hooks are implemented (in #678): both pre-build hooks and paths to Dockerfiles are supported.