Docker progress output hides degugging output from pre-build hooks

[ydirson]

cross version used: 44011c8

With docker 23.x from docker.com, the heavy progress output from Docker contains many terminal escape sequences, and they interfere with output we can print from a pre-build hook, as much as hiding a 200+ line find output like generated from:

[target.x86_64-unknown-freebsd]
pre-build = ["""
bash -c ". /freebsd-install.sh && install_freebsd_package xen-tools" &&
find /usr/local/x86_64-unknown-freebsd12/  -name "*xen*"
"""]

Workaround: redirect to a file so docker stops producing terminal control sequences, but at the same time modify the pre-build hook so that docker does not just fetch an overlay from cached previous runs.

[Emilgardis]

one way around this is setting --progress=plain

CROSS_BUILD_OPTS=--progress=plain