Shell out to docker buildx build to build images

[yuvipanda]

pydocker uses the HTTP API (equivalent to docker build) which has been dead for ages. docker buildx (the interface to BuildKit) is what we should be using, but alas pydocker does not currently support it and I suspect it never will (docker/docker-py#2230).

This PR simply shells out, as I think that's the way.

I'm going to list some intentional choices I've made here.

1. Intentionally replace the build functionality of the existing DockerEngine than make a new one. The old way is deprecated and we should not support it.
2. Continue using dockerpy for all other non-build operations. It works fine and I don't see a need to change it right now.
3. Simply extract the tarball we construct elsewhere. This is kinda double work, but we can treat this as a possible optimization pathway in the future.
4. Currently ignore container_limits (because buildx doesn't support it, it should be set at the level of the builder instead)

Questions to answer

• What do we do about container_limits here?
• We probably need to deprecate / throw errors on some existing traitlets as they won't really have any effect here. What traitlets are those?
• We should treat this as a breaking change and make a release after, right?

Traitlets to deprecate

• Repo2Docker.build_memory_limit - this must be instead set up by the builder
• Repo2Docker.extra_build_args - as this is purely about pydocker's build method that we no longer use

Fixes #875

[yuvipanda]

This isn't complete, putting it up here to see how many tests fail.

[minrk]

Nice! This will also unblock using COPY --chmod as in #1395

Re: extra_build_args, presumably we do want to keep an equivalent passthrough for extra args to pass to buildx build on the command-line (a list this time)?

[minrk]

Looks like rm and forcerm don't have equivalents. Just noting that this is intentional. Are there likely to be significant changes in the cache behavior with buildkit we need to pay attention to?

[yuvipanda]

@minrk I added extra_buildx_build_args in what feels like the right place for it. Will need to figure out a test.

[minrk]

If you want to specify the type of items in the list, give a single TraitType instance here (not list):

Suggested change

	[],
	Unicode(),

[minrk]

Will need to figure out a test.

iidfile looks like it might be a good arg to test with, since it just creates a file to check. Or --check appears to skip the build entirely, so should be quick.

[minrk]

Here's a small test for extra buildx args with --check that goes really quick since it doesn't actually build the image:

python3 -m repo2docker --no-run '--DockerEngine.extra_buildx_build_args=["--check"]' .

which outputs (among other things):

Check complete