Move Dockerfile parsing to a dedicated package, add support for RUN --mount=type=bind,from=...

[tianon]

Also, add a bunch of test cases

(made as two commits so it's hopefully easier to review - one that's move + tests, one that's new functionality)

[tianon]

Before this PR: (statements) 84.9%
After this PR: (statements) 85.2%

That only counts code with tests, so I've managed enough coverage here to get a net increase in the covered code even with the added lines. 💪

[tianon]

Now with even more coverage!!

-(statements)			84.9%
+(statements)			85.6%

-github.com/docker-library/bashbrew/pkg/dockerfile	0.032s	coverage: 87.1% of statements
+github.com/docker-library/bashbrew/pkg/dockerfile	0.026s	coverage: 91.4% of statements

[tianon]

-(statements)			84.9%
+(statements)			86.3%

-github.com/docker-library/bashbrew/pkg/dockerfile	0.026s	coverage: 91.4% of statements
+github.com/docker-library/bashbrew/pkg/dockerfile	0.027s	coverage: 97.1% of statements

[tianon]

Here's a snapshot:

Those are, in order:

• an impossible codepath (I can't think of any way to reach it -- suggestions welcome!)
• an error in reading the Dockerfile (which in all our tests comes from a string in memory, so there's no way it could possibly error)

[tianon]

• an impossible codepath (I can't think of any way to reach it -- suggestions welcome!)

Nerdsniped myself and thought of a way, but it is technically a bug in my parsing of continuation lines, which is now fixed (the previous implementation might've very harmlessly ignored a line whose "instruction" was literally "\" and now it skips that line as empty correctly).

[tianon]

-(statements)			84.9%
+(statements)			86.5%

-github.com/docker-library/bashbrew/pkg/dockerfile	0.027s	coverage: 97.1% of statements
+github.com/docker-library/bashbrew/pkg/dockerfile	0.026s	coverage: 98.7% of statements

[tianon]

Simplify pkg/dockerfile interface by ditching pointer

This means slightly more typing in "zero-value" cases (nil vs dockerfile.Metadata{}), but the tradeoff is that it's simpler to use and reason about (and all the struct members are pointer-type map/slice values anyhow, so copying the struct is still pretty cheap).

This also swaps the scanner error handling to return the partially parsed Metadata object alongside the scanner error -- the error already tells us the object isn't fully complete data, so it's fair/fine to return and will likely just be ignored by the caller instead. This also allows us to get to 100% code coverage. 👀

This also updates our "treat oci-import just like FROM scratch" code to actually parse FROM scratch so we can't accidentally cause "missing data" bugs there in the future, and I implemented that using sync.OnceValues which requires upgrading to Go 1.21, but IMO that's a worthwhile tradeoff (because sync.OnceValues makes that code so clean/simple).

-(statements)			84.9%
+(statements)			86.7%

[tianon]

https://gha.dag.dev/trace?artifact=2410423127&file=coverage.html&name=coverage&size=22577&uri=https%3A%2F%2Fgithub.com%2Fdocker-library%2Fbashbrew%2Factions%2Fruns%2F12700849704%2Fjob%2F35404320284#file3 if you want to browse the coverage for yourself (time-limited link, because GHA and their artifacts are inherently time-limited)