Simulate json output from docker buildx build

[yuvipanda]

We still use the docker python api to push images, and to run. So we need to set string_output to false, and simulate the json output on buildx. Alternatively we could completely get rid of the python API, but that's a much bigger lift (especially around registry credentials) that I don't want us to do right now.

Without this, push progress is reported as raw bytes in the binderhub UI

[minrk]

Is there a simple test that would have caught this?

I was able to test manually by running a local registry:

docker run --rm 127.0.0.1:9000:5000 registry

and then pushing to it:

python3 -m repo2docker --no-run --json-logs --image-name=localhost:9000/test/image .

before:

{"message": "Reusing existing image (localhost:9000/test/image), not building."}
{"message": "b'{\"status\":\"The push refers to repository [localhost:9000/test/image]\"}\\r\\n'", "phase": "pushing"}
{"message": "b'{\"status\":\"Preparing\",\"progressDetail\":{},\"id\":\"9ffc2f2e8fb8\"}\\r\\n{\"status\":\"Preparing\",\"progressDetail\":{},\"id\":\"d12bcb450636\"}\\r\\n{\"status\":\"Preparing\",\"progressDetail\":{},\"id\":\"c20414e1266b\"}\\r\\n{\"status\":\"Preparing\",\"progressDetail\":{},\"id\":\"6838097029a0\"}\\r\\n{\"status\":\"Preparing\",\"progressDetail\":{},\"id\":\"3ec51339a507\"}\\r\\n'", "phase": "pushing"}

after:

{"message": "View build details: docker-desktop://dashboard/build/multi/multi0/ys76161hbgd0qn0clt5vfngly\n", "phase": "building"}
{"message": "Successfully pushed localhost:9000/test/image:1739524755", "phase": "pushing"}

but I'm not sure how to test this practically without launching a local registry for the test.

[yuvipanda]

Seems to work fine with this!