use multiarch/qemu-user-static docker image for registering qemu

kokoro/linux/aarch64/qemu_helpers/prepare_qemu.sh

@@ -10,21 +10,17 @@ cat /proc/sys/fs/binfmt_misc/qemu-aarch64
 
 # Kokoro ubuntu1604 workers have already qemu-user and qemu-user-static packages installed, but it's and old version that:
 # * prints warning about some syscalls (e.g "qemu: Unsupported syscall: 278")
-# * doesn't register with binfmt_misc with the "F" flag we need (see below)
-# To avoid the "unsupported syscall" warnings, we download a recent version of qemu-aarch64-static.
-# On newer distributions (such as glinux), simply "apt install qemu-user-static" is sufficient.
-curl -sSL --output qemu-aarch64-static https://github.com/multiarch/qemu-user-static/releases/download/v5.2.0-2/qemu-aarch64-static
-chmod ugo+x qemu-aarch64-static
-sudo mv qemu-aarch64-static /usr/local/bin
-/usr/local/bin/qemu-aarch64-static --version
-
-# register qemu-aarch64-static with binfmt_misc to use it as emulator for aarch64 binaries (based on their magic and mask)
-# The format of the configuration string is ":name:type:offset:magic:mask:interpreter:flags"
-# The most important flag is "F", which allows the qemu-aarch64-static binary to be loaded regardless of chroot and namespaces.
+# * doesn't register with binfmt_misc with the persistent ("F") flag we need (see below)
+#
+# To overcome the above limitations, we use the https://github.com/multiarch/qemu-user-static
+# docker image to provide a new enough version of qemu-user-static and register it with
+# the desired binfmt_misc flags. The most important flag we need is "F" (set by "--persistent yes"),
+# which allows the qemu-aarch64-static binary to be loaded eagerly at the time of registration with binfmt_misc.
 # That way, we can emulate aarch64 binaries running inside docker containers transparently, without needing the emulator
-# to be accessible from the docker image we're emulating.
-echo -1 | sudo tee /proc/sys/fs/binfmt_misc/qemu-aarch64  # delete the existing registration
-echo ":qemu-aarch64:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xb7\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/local/bin/qemu-aarch64-static:OCF" | sudo tee /proc/sys/fs/binfmt_misc/register
+# binary to be accessible from the docker image we're emulating.
+# Note that on newer distributions (such as glinux), simply "apt install qemu-user-static" is sufficient
+# to install qemu-user-static with the right flags.
+docker run --rm --privileged multiarch/qemu-user-static:5.2.0-2 --reset --credential yes --persistent yes
 
 # Print current qemu reqistration to make sure everything is setup correctly.
 cat /proc/sys/fs/binfmt_misc/qemu-aarch64