Add --metadata-file flag to output build metadata

[rittneje]

Output build metadata (such as image digest) to a file based on a flag to buildctl.
Fixes #1158.

buildctl build --frontend=dockerfile.v0 --metadata-file meta.json --local context=. --local dockerfile=.. --output type=image,name=foo:latest,push=false

{"containerimage.config.digest":"sha256:9ba453cfcf085ebe34d7e3bc0cd13e02038361693a53d8663ddca8f83e26447d","containerimage.digest":"sha256:475f3a5ec8513c157acc20aacf3c872afb3b508ed40f9e9fb519d31897b7a3f0","image.name":"foo:latest"}

@tonistiigi @AkihiroSuda This is based on the comments from #1315.

[tonistiigi]

LGTM but can you add a test. You can also modify an existing test and add an extra check.

[rittneje]

@tonistiigi Sure I can try to add something. Also, do you understand the test failure here? Not sure how these changes would have affected it.

        --- FAIL: TestClientGatewayIntegration/TestClientGatewayExecError/worker=containerd/only_rootfs (1.10s)
            build_test.go:1216: 
                	Error Trace:	build_test.go:1216
                	Error:      	Not equal: 
                	            	expected: "su6iub2plufj65normatzcstv"
                	            	actual  : ""
                	            	
                	            	Diff:
                	            	--- Expected
                	            	+++ Actual
                	            	@@ -1 +1 @@
                	            	-su6iub2plufj65normatzcstv
                	            	+
                	Test:       	TestClientGatewayIntegration/TestClientGatewayExecError/worker=containerd/only_rootfs

[rittneje]

@tonistiigi Actually, I took a look at the existing unit tests. They are pretty complicated, so I'm not entirely sure what I'm supposed to do. Can you provide some additional guidance on this?

[tonistiigi]

You can base it on https://github.com/moby/buildkit/blob/master/client/client_test.go#L1783-L1821 for example. No need to push in your case.

[tonistiigi]

On second look, I wonder though if client pkg is the correct place for it. Solve already returns this value so it is kind of duplicated to have side-effect and write it to a file in there as well. I think it might be more suited directly in buildctl pkg.

[rittneje]

@tonistiigi Apologies for the churn. I was not able to run the tests locally before due to docker/cli#2989. The test I added is passing now.