podman complains about shell directive:
SHELL is not supported for OCI image format, [/bin/bash -o pipefail -c]
will be ignored. Must use `docker` format

Alas, there is no official ARMv7 bindist for 8.8.3, which caused the build to fail.

This patch makes GHC 8.8 the minimum bootstrap compiler across all
Docker images in preparation for the same requirement being imposed
on GHC. In most cases, I was able to use 8.8.3. The exception is
`x86_64-linux-deb10`, where the only 8.8 bindist I could find was
8.8.1.