docker: install qemu from backports #861

Job	Run time
Check DockerHub secrets exist	1s
test (arch)	1m 0s
test (bookworm)	55s
test (trixie)	1m 2s
Build Docker container	3m 21s
recipes on nofakemachine	33s
templating on nofakemachine	56s
Run unit tests	1m 55s
debian (amd64) on nofakemachine	1m 45s
recipes on qemu	46s
templating on qemu	49s
partitioning on qemu	1m 6s
msdos partitioning on qemu	57s
debian (amd64) on qemu	5m 57s
recipes on uml	1m 0s
templating on uml	57s
partitioning on uml	1m 14s
msdos partitioning on uml	1m 15s
debian (amd64) on uml	4m 57s
recipes on kvm	1m 2s
templating on kvm	37s
partitioning on kvm	42s
msdos partitioning on kvm	38s
debian (amd64) on kvm	1m 9s
debian (arm64) on kvm	7m 50s
debian (armhf) on kvm	2m 52s
arch on kvm	1m 37s
apertis on kvm	2m 9s
allgreen	1s
Publish to GHCR	1s
Publish to DockerHub	1s
	49m 5s

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