Add on_start label for running a command every time the container starts

[deansheather]

Adding the on_open label to an image will cause sail to exec that command inside of the container's project directory every time it creates a container.

Added the new label to the documentation.

When this was discussed in #191, the concept was a .sail/on_open file rather than a label. This PR uses a label instead of a file, as (in my opinion) a label seems to be a more "sail way" of doing this. As using a label for this functionality hasn't been discussed, this PR should not be merged until this is discussed.

Emulating the .sail/on_open file is as simple as LABEL on_open ".sail/on_open", and making sure that .sail/on_open has a shebang and is executable.

Closes #191.

[deansheather]

Still need to write tests for this before this can be merged, but I'd like to hear any thoughts before doing so.

[deansheather]

Maybe this label should be changed to on_start.

[nathanpotter]

This looks good if you want to start working on tests for it.

I agree, I think we should name it on_start, seems to be a bit more clear.

[ammario]

on_start definitely makes more sense

[deansheather]

Is the only way to write tests for this to make a new GitHub repository with an on_start label?

[nathanpotter]

To test it, you could add an example hat showing how to use the label, maybe just create a file in the container or something, then use that example hat in a test and validate that whatever changes the on_start command did actually occurred after the container is started

[deansheather]

I'm having an issue running the tests on my machine if I run all of them at the same time. Almost every time a random Test_runner test will fail while trying to get the code-server port:

--- FAIL: Test_runner/BaseImageHat (29.99s)
    --- PASS: Test_runner/BaseImageHat/Labels (0.01s)
    --- PASS: Test_runner/BaseImageHat/CodeServerStarts (0.11s)
    --- FAIL: Test_runner/BaseImageHat/FromContainer (5.45s)
        require.go:794: 
                    Error Trace:    runner_test.go:68
                    Error:          Received unexpected error:
                                    failed to find code server port:
                                        go.coder.com/sail.runnerFromContainer
                                            /home/dean/git/github.com/cdr/sail/runner.go:495
                                      - failed while trying to find code-server port:
                                        go.coder.com/sail.codeServerPort
                                            /home/dean/git/github.com/cdr/sail/codeserver.go:184
                                      - failed to find port:
                                        go.coder.com/sail/internal/codeserver.init.ializers
                                            /home/dean/git/github.com/cdr/sail/internal/codeserver/proc.go:15
                    Test:           Test_runner/BaseImageHat/FromContainer

If I run tests with -test.parallel 1 it succeeds. Should we disable parallel testing for now?

[deansheather]

@nathanpotter Could you please approve the tests?