Containerfile.5.md should document "magic" build-args

[cevich]

Podman and buildah (I believe) both support similar/same set of automatic build-args as in the Docker documentation (I'm guessing, so maybe not all are supported?):

• TARGETPLATFORM - platform of the build result. Eg linux/amd64, linux/arm/v7, windows/amd64.
• TARGETOS - OS component of TARGETPLATFORM
• TARGETARCH - architecture component of TARGETPLATFORM
• TARGETVARIANT - variant component of TARGETPLATFORM
• BUILDPLATFORM - platform of the node performing the build.
• BUILDOS - OS component of BUILDPLATFORM
• BUILDARCH - architecture component of BUILDPLATFORM
• BUILDVARIANT - variant component of BUILDPLATFORM

In either case, some/all of these should be documented in the Containerfile man page. There are many situations where (esp) knowing (for example) the BUILDARCH is useful in RUN commands. Importantly because it's golang-flavored and difficult to access elsewhere (i.e. uname -m is different).

[rhatdan]

Interested in opening a PR?

[cevich]

I must admit I'm tempted to, I'm simply afraid I may not be able to get to it soon-ish. I'll put it on my TODO list, but if anybody else is inspired to fix this they should go ahead.