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Explain binfmt_misc requirements 

Getting the QEMU interpreters to work transparently inside containers seems to be a bit of a sticking point, with major distributions still on kernels and/or binfmt-support package versions that don't support the fix_binary flag yet. Give the reader a little more guidance what to look out for.
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Artur Klauser 2020-01-04 00:00:40 +01:00 committed by GitHub
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@ -116,7 +116,7 @@ When invoking a build, the `--platform` flag can be used to specify the target p
Multi-platform images can be built by mainly three different strategies that are all supported by buildx and Dockerfiles. You can use the QEMU emulation support in the kernel, build on multiple native nodes using the same builder instance or use a stage in Dockerfile to cross-compile to different architectures.
QEMU is the easiest way to get started if your node already supports it (e.g. if you are using Docker Desktop). It requires no changes to your Dockerfile and BuildKit will automatically detect the secondary architectures that are available. When BuildKit needs to run a binary for a different architecture it will automatically load it through a binary registered in the binfmt_misc handler.
QEMU is the easiest way to get started if your node already supports it (e.g. if you are using Docker Desktop). It requires no changes to your Dockerfile and BuildKit will automatically detect the secondary architectures that are available. When BuildKit needs to run a binary for a different architecture it will automatically load it through a binary registered in the binfmt_misc handler. For QEMU binaries registered with binfmt_misc on the host OS to work transparently inside containers they must be registed with the fix_binary flag. This requires a kernel >= 4.8 and binfmt-support >= 2.1.7. You can check for proper registration by checking if `F` is among the flags in `/proc/sys/fs/binfmt_misc/qemu-*`.
Using multiple native nodes provides better support for more complicated cases not handled by QEMU and generally have better performance. Additional nodes can be added to the builder instance with `--append` flag.