Add dockerfile for cross.

[Alexhuszagh]

This uses the official Docker docker-in-docker image for the latest stable, and installs the latest stable Rust version internally. It uses an entrypoint to ensure cargo is on the path, and that cross is installed from the latest git (locked), which when tagged will work well with our releases. It exports the environment variable CROSS_CONTAINER_IN_CONTAINER so everything should work as expected.

Closes #468.
Replaces #667.

[Alexhuszagh]

For some reason, the tags aren't working:
https://github.com/cross-rs/cross/runs/7082679487?check_suite_focus=true#step:5:13

I might have to override some of the envvars.

[Alexhuszagh]

bors try --target cross

[Alexhuszagh]

I managed to get this to work by effectively skipping the test if it was a Docker image for cross, but that's definitely not the solution here. Any suggestions would be great, since my idea was to skip testing if the matrix.target == 'cross', but I didn't have any luck with:

• ${{ matrix.target != 'cross' }}
• ${{ matrix.target }} != 'cross'

[Emilgardis]

Does this entry point approach not incur quite a bit of startup?

Also, this doesnt test the code in the branch or even local base, but current main

[Emilgardis]

I managed to get this to work by effectively skipping the test if it was a Docker image for cross, but that's definitely not the solution here. Any suggestions would be great, since my idea was to skip testing if the matrix.target == 'cross', but I didn't have any luck with:

* `${{ matrix.target != 'cross' }}`

* `${{ matrix.target }} != 'cross'`

Instead of doing that, better approach I think would be to make something similar to

cross/xtask/src/ci.rs

Line 70 in eb2b8ad

gha_output("has-image", "true")

, and gate the test on that

The first alternative is correct also, however in if statements you dont need ${{}}

[Alexhuszagh]

Yeah I probably need another test script or something. Also, it's probably a good idea to move this to Ubuntu, since all our other images are. I think having a single Alpine image probably isn't worth it. It's also a much larger image since it needs a redundant x86_64-unknown-linux-musl as a result.

Maybe the integration tests for docker-in-docker could be based off of this?

[Alexhuszagh]

I changed the Dockerfile context to the workspace root, and then use that in the build process, so we can copy over the entire project over from the current source code, to ensure the image reflects the current project state.

bors try --target cross

[Emilgardis]

LGTM!

One thing missing is changing the label for title, it should probably be simply "cross" or similar

[Alexhuszagh]

bors try --target cross

[bors]

try

Build succeeded:

• conclusion

[Emilgardis]

lgtm!

[Alexhuszagh]

bors r=Emilgardis

[bors]

Build succeeded:

• conclusion