Support building Dockerfiles that mount secrets as environment variables

[carles-garcia]

Feature request description

Since Dockerfile 1.10 the Docker Engine can mount secrets as environment variables specified in the RUN command.

https://docs.docker.com/build/buildkit/dockerfile-release-notes/#1100

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

It would be useful for podman build to support it. It's already possible using podman run.

[rhatdan]

@flouthoc any chance you could look at this?

[flouthoc]

I'll take a look at this.