Add the --arch and --arch-variant build options

[afbjorklund]

Description of the Pull Request (PR):

Documents the --arch and --arch-variant options for pull and for build.

Currently variant is only used on arm32, not on amd64 and not on arm64.

The default in docker and in kubernetes is to use arm32v7, and arm64v8.

https://github.com/docker-library/official-images#architectures-other-than-amd64

For amd64 the default is v1, but v2 and v3 are now used by RHEL 9 and 10.

For arm64 the default is v8, but v9 is now getting more common as well...

But currently these architectures do not support (and do not need) any variants.

This fixes or addresses the following GitHub issues:

• Fixes Document --arch and --arch-variant for build #325

[afbjorklund]

I opened another issue, to make the --arch-variant option obsolete - by having it just default to 7.

• Consider defaulting arm architecture to the v7 variant apptainer#2807

Also wondered, if the --arch-variant options should be written as v7 and v8 (instead of 7 and 8)?

The apptainer commands already accept both forms.

[DrDaveD]

I still see one thing missing. The CPU emulation section is in this file but it only talks about running a container. Somewhere there needs to be something that says that CPU emulation needs to be enabled in order to build for different architectures. I think it probably makes most sense in the new Build architecture section but I leave that up to you.

[afbjorklund]

There is potentially some duplication (for now I just linked the other two places to the one with the information).

And not all image builds actually do require emulation, but most of them do (depending on scripts vs. just files)

EDIT: I guess technically I only linked "specifying an architecture", not the "CPU emulation" chapter below it...

Perhaps we need some common section outside either command, and then link both pull and build to it?

[DrDaveD]

Also wondered, if the --arch-variant options should be written as v7 and v8 (instead of 7 and 8)?

The apptainer commands already accept both forms.

I don't have an opinion on that, I'll leave it up to you.

[afbjorklund]

Added links to both sections, and examples of both. Hopefully the --arch-variant will soon be obsolete.

• Consider defaulting arm architecture to the v7 variant apptainer#2807

The names of the variants in the documentation should follow the code, so when that changes it will too...

supported variants are [5, 6, 7] (changed to supported variants are [v5, v6, v7])