templating support for definition file

[preminger]

Description of the Pull Request (PR):

Provides support for --build-arg and --build-arg-file to pass variables to build process, allowing definition file to use values using {{ argname }} strings. Also supports an %arguments section in definition file to provide default values for the arguments used by the definition file, which can then be overridden via the aforementioned command-line options.

This is a pick of apptainer/apptainer#1356

[dtrudg]

I like the approach to the replacement here, vs what's in the initial cherry pick 👍.

Generally, the more I look at this, the more I think there need to be some researched and carefully considered design decisions made... which may or may not mesh with the Apptainer implementation. From an initial reading:

• What should be the restrictions on build arg names?
• Should there be a way to escape the {{ ... }} pattern in a def file? Some programs will take arguments in that format. It's not uncommon for Go programs to take Go template snippets to format output.

We also need to carefully consider what happens with remote builds. Is this parsing on the client side (okay)... or will the def currently be sent up to the remote builder before the replacements occur?

[dtrudg]

Lookst prety good to me at this point. Definitely prefer the code as re-written.

I think It'd be good for a docs PR to be written up in parallel with this WIP PR, so that they can be merged together. The reason for this is that it'll focus some attention on what key/val pairs are allowed, and how things like quoting etc. differ between CLI and file. I'm not convinced there aren't some rough edges there that we might want to address, yet.

[dtrudg]

LGTM ... This is a ✅ just held in order to raise it with @EmmEff tomorrow, just to 100% confirm we are all good with the exact change in types.Definition

[dtrudg]

Just thought.... this only works for local builds, at present, right? If that's the case we should have a warning/error if used with --remote. We don't need remote support to merge at this time.