gateway: enable named contexts for gateway frontend

[tonistiigi]

This allows overwriting the definition of #syntax names same as can be done with other dockerfile names.

Signed-off-by: Tonis Tiigi tonistiigi@gmail.com

[jedevc]

I think we might need to modify some of the caps detection logic as well - I gave this a spin on a frontend that doesn't have the moby.buildkit.frontend.contexts cap, and I get the following:

$ docker buildx build . -f Testfile --build-context custom-frontend=oci-layout://my-custom-frontend
...
ERROR: failed to build: current frontend does not support --build-context. Named contexts are supported since Dockerfile v1.4. Use #syntax directive in Dockerfile or update to latest BuildKit.

Even though my frontend doesn't use build-contexts, because of the propagation of build-contexts, the frontend needs to declare the cap.

[tonistiigi]

I think we might need to modify some of the caps detection logic as well - I gave this a spin on a frontend that doesn't have the moby.buildkit.frontend.contexts cap, and I get the following:

Not sure I understand. Isn't it correct that if the user passes --build-context and the frontend doesn't support it, then it is an error? Isn't this the behavior before this PR as well?

[jedevc]

Because with this patch we can determine the frontend based on the build-context, if you want to use this feature to only change the frontend, that frontend must support build-contexts, or it's propagated through, and immediately errors.

[tonistiigi]

I think it is just something we need to accept that if you wish to use --build-context with #syntax to switch frontends, the frontend itself needs to support it as well. The only other way would be to start passing 2 different sets of contexts (one for gateway and another for redirect) what will get messy quickly.

[jedevc]

LGTM, but can we also get a test 👀

[tonistiigi]

Testing this would mean building a custom frontend inside the test. Gateway testing is specific to Dockerfile frontend atm and is built into the test script that loads the frontend image.