Support Make variable substitution in `oci_push`

[ouillie]

This would allow, among other thing, configuring the registry using a command-line flag, e.g.:

load("@bazel_skylib//rules:common_settings.bzl", "string_flag")

string_flag(
    name = "registry",
    make_variable = "REGISTRY",
    build_setting_default = "localhost:5000",
)

oci_push(
    name = "image-push",
    image = "//some:image",
    remote_tags = ["latest"],
    repository = "$(REGISTRY)/image-name",
    toolchains = [":registry"],
)

Then you could run:

bazel run //package:image-push --//package:registry=docker.io/library

Implementing this is just a matter of running ctx.expand_make_variables.

[ouillie]

Upon further inspection, I see there is already a --repository flag that the push script will try to parse, so my original motivating example is already covered, although perhaps somebody can think of another reason to go with the more-flexible Make variables approach. I think this can be de-prioritize for now, though.