feat: support --mount=type=secret,id=foo,env=bar

[aeijdenberg]

Allow secrets specified with mount to result in env variable set as an alternative to mounting a file.

This matches Docker behaviour:
https://docs.docker.com/reference/dockerfile/#example-mount-as-environment-variable

What type of PR is this?

/kind feature

What this PR does / why we need it:

Docker supports making secret mounts appear as environment variables during a RUN command. This adds the same to buildah.

See: https://docs.docker.com/reference/dockerfile/#example-mount-as-environment-variable

How to verify it

bats test added.

Which issue(s) this PR fixes:

Fixes #5892

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Adds support for mounting a secret to an environment variable. For example:

RUN --mount=type=secret,id=mysecret,env=FOO sh -c 'echo "Hello $FOO"'

[openshift-ci]

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[filip-be]

Hi Team!
What's the status of this PR? is it going to be updated and merged anytime soon or is there any pending issue that needs to be resolved?

[aeijdenberg]

Rebased on latest main to fix merge conflict in test.

[github-actions]

A friendly reminder that this PR had no activity for 30 days.

[ab-gh]

Hi there, has there been any progress in the status of this PR? Not having docker-like type=secret,env=bar is really hindering some progress.

[aeijdenberg]

Rebased to fix merge conflict.

[aeijdenberg]

Rebased to fix merge conflict.