Add support for optional --buildkitd-config flag to buildx builder function

[SarahB-hub]

What:

• Adds optional --buildkitd-config flag to the docker buildx create function cmdSetupBuildx(), passable with PLUGIN_BUILDER_CONFIG envvar

Why:

Builders using the docker-container engine do not inherit some daemon config such as registry mirrors from the Docker daemon in their origin layer, and instead use a config file handed through a flag in their docker create buildx command - Source

These changes allow users to optionally provide a PLUGIN_BUILDER_CONFIG envvar with a path to a Buildkit daemon config file, so registry mirrors can be added to docker-container builders.

Note:

• The flag --config is specified in configuring Buildkit registry mirrors but appears to be outdated, so I've added --buildkitd-config instead as it's documented both here and in the flag list of the latest version docker create buildx --help

[jamie-harness]

Thanks for contributing. The change looks good. We can create a release after your merge within the week

[SarahB-hub]

Thanks for contributing. The change looks good. We can create a release after your merge within the week

I don't have authorisation to Merge it I'm afraid! I think someone on your team might need to :)