* Avoid race condition when fetching containers during build
So far only a single builder was active for each architecture. This
toghether with the naming scheme to include architecture/machine name
made sure that an image could only be fetched or used by a single
builder.
However, since most systems are now aarch64, multiple runners are now
active for a single architecture. This makes it necessary to lock
fetching/coping of container images to avoid race conditions.
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