Building for arm64 with `docker buildx` hangs on libx265 regression

[nine9ths]

I've checked out the 7.1.1 tag and I'm attempting to build for both amd64 and arm64 with the following command:

docker buildx build --platform linux/amd64,linux/arm64 \
    -t a-private-repository.com/static-ffmpeg:7.1.1 \
    --push \
    --output=type=registry,registry.insecure=true \
    --provenance=false \
    -f Dockerfile .

It appears to hang indefinitely (it timed out after 24hrs) on this final step while doing the arm64 build (the amd64 build completed):

RUN ["/ffmpeg", "-f", "lavfi", "-i", "testsrc", "-c:v", "libx265", "-t", "100ms", "-f", "null", "-"]

I've tried it on two different Ubuntu 20.04 host machines, the second one has over 300gb of memory so I don't think it's a resource issue.

Any ideas what might be the problem?