Docs: fix build arg syntax for BuildKit

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Problem to solve

There is an error in the documentation at https://docs.gitlab.com/ci/docker/using_buildkit/.

The syntax for giving build args with BuildKit is not --build-arg foo=bar but --opt build-arg:foo=bar

Further details

See BuildKit documentation: https://github.com/moby/buildkit/blob/7cf577a2f736ced2fa453226f3f2bb5f83648e85/README.md?plain=1#L231

Proposal

- --build-arg http_proxy=$http_proxy \
- --build-arg https_proxy=$https_proxy \
- --build-arg no_proxy=$no_proxy \
+ --opt build-arg:http_proxy=$http_proxy \
+ --opt build-arg:https_proxy=$https_proxy \
+ --opt build-arg:no_proxy=$no_proxy \

Who can address the issue

I wanted to, but I can't make a commit to my repository fork: