docker: Use prebuild archlinux keyring #739

Job	Run time
Check DockerHub secrets exist	1s
test (arch)	1m 10s
test (bookworm)	54s
test (trixie)	1m 4s
Build Docker container	3m 3s
Run unit tests	2m 2s
recipes on nofakemachine	44s
templating on nofakemachine	1m 48s
debian (amd64) on nofakemachine	1m 15s
recipes on qemu	56s
templating on qemu	55s
partitioning on qemu	53s
msdos partitioning on qemu	55s
debian (amd64) on qemu	6m 29s
recipes on uml	40s
templating on uml	39s
partitioning on uml	54s
msdos partitioning on uml	1m 7s
debian (amd64) on uml	4m 54s
recipes on kvm	42s
templating on kvm	46s
partitioning on kvm	46s
msdos partitioning on kvm	38s
debian (amd64) on kvm	1m 19s
debian (arm64) on kvm	2m 16s
debian (armhf) on kvm	2m 41s
arch on kvm	1m 48s
apertis on kvm	1m 43s
Publish to GHCR	2m 17s
allgreen	1s
Publish to DockerHub	2m 25s
	47m 45s

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