docker: install qemu-user-static from backports #880

Job	Run time
Check DockerHub secrets exist	1s
test (arch)	1m 6s
test (bookworm)	51s
test (trixie)	53s
Build Docker container	24m 25s
Run unit tests	1m 50s
recipes on nofakemachine	37s
templating on nofakemachine	35s
debian (amd64) on nofakemachine	58s
recipes on qemu	1m 2s
templating on qemu	47s
partitioning on qemu	1m 20s
msdos partitioning on qemu	56s
debian (amd64) on qemu	5m 5s
recipes on uml	1m 23s
templating on uml	1m 9s
partitioning on uml	1m 0s
msdos partitioning on uml	59s
debian (amd64) on uml	4m 59s
recipes on kvm	47s
templating on kvm	37s
partitioning on kvm	39s
msdos partitioning on kvm	35s
debian (amd64) on kvm	1m 14s
debian (arm64) on kvm	2m 34s
debian (armhf) on kvm	2m 27s
arch on kvm	2m 8s
apertis on kvm	1m 51s
allgreen	4s
Publish to GHCR	0s
Publish to DockerHub	0s
	1h 2m 52s

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