Add FTL build template

[imjasonh]

Define a reusable build template for images built using the FTL model for container image construction.

This could be a single FTL template, or a template for each supported language (NodeJS, Python, PHP), whichever is simpler for users and maintainers. This is also an opportunity to document what FTL "is", how it works, why it's good, etc.

This repo could also be a home for the FTL builder image's source, making ftl.par available as a builder image usable in on-cluster builds. We could even move ftl.par's source to this repo, if that's preferrable.

This repo could also present an opportunity to rewrite FTL in Go, using the go-containerregistry library.

/cc @aaron-prindle @dlorenc

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