Add export to Gateway API

[vladaionescu]

I'd like to be able to perform an export of an image tar in the middle of the build, and then use that tar in subsequent LLB operations. The image tar doesn't need to be streamed to the client - it just needs to be available to be llb.Copy'd or mounted in llb.Run commands.

Use-case

For docker-in-buildkit: construct an image that is then used in a docker run command that executes within the build.

Workaround

Use separate consecutive solves. This has the disadvantage that it is usually slow at scale (noticeable at 3+ images) if done sequentially. Parallel may be possible, but makes the client code very complicated often.

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Happy to take a look into implementing this myself, if this turns out to be not extremely complicated. Could use architectural guidance as I believe @tonistiigi has done previous thinking to this end.

CC @tiborvass @hinshun @shykes

[shykes]

I have requested something similar (in my case I need to export to a docker registry as part of a solve). To paraphrase @tiborvass and @hinshun , the current approach is to NOT add an export to LLB, but instead add it to the gateway API, which allows frontends to export within the context of a single solve.

[tiborvass]

My brain is absolutely not working. Yes, gateway API is what I wanted to say... :(

[vladaionescu]

Gateway API is not ideal for our use-case, as the transfer of images gets noticeably slow at scale, even if it takes place on the same host.

But I can see the benefit of having a gateway API for other use-cases, like performing exports for multiple images within a single build. This is an issue we have previously worked around by writing our own exporter and abusing the refs (meant for multi-platform) to use them for the different images that need exporting.