Fix image build errors with podman and nerdctl.

[Alexhuszagh]

Ensures that docker and nerdctl, which support the custom --output and --cache-from flags use them, while podman and unknown container engines use the strict, minimal subset podman supports. This is because podman only supports a registry/repository, without a tag, for --cache-from, which means it synchronizes poorly with our target subs, like centos. Therefore, unless unsupported, we should always use the features available for our container engine.

This also fixes the --pull flag on nerdctl, which is unsupported. Engines without BuildKit extensions by default, such as nerdctl, now do not use buildx unless explicitly enabled with CROSS_CONTAINER_ENGINE_NO_BUILDKIT=0.

Closes #1031.

[Alexhuszagh]

bors try --target x86_64-unknown-linux-gnu

I've added a custom image build to the Podman test (it's a trivial pre-build step, so it just tests the image build steps work), so we can ensure building and testing images work.

[bors]

try

Build succeeded:

• conclusion

[Alexhuszagh]

I haven't added tests for Lima (nerdctl), since it requires a Homebrew installation which takes a long, long time and sometimes errors due to too many file descriptors being open. I have, however, tested this locally and confirmed the above flags work. We're currently waiting on lima-vm/lima#1075 to see if we can get BuildKit working with Lima.

[Alexhuszagh]

We might also want to default to no buildx with Lima, since it doesn't support it.

[Emilgardis]

I feel like this should be an error, i.e it should not be possible to specify no output and then not have it happen

[Alexhuszagh]

I've changed it so we error if it doesn't support the output flag, with another branch:

if push {
    docker_build.arg("--push");
} else if engine.kind.supports_output_flag() && no_output {
    docker_build.args(["--output", "type=tar,dest=/dev/null"]);
} else if no_output {
    msg_info.fatal("cannot specify `--no-output` with engine that does not support the `--output` flag", 1);
} else if has_buildkit {
    docker_build.arg("--load");
}

[Emilgardis]

lgtm!

bors r+

[bors]

Build succeeded:

• conclusion