Trying to build the images for different build platforms using the
Makefile isn't possible without changing the code.
This change exposes the PLATFORM and TAG variables to enable the user to
build and tag the images with greater flexibility.
The same TAG variable can be used on the prod-*-run targets too.
Postgres is not built in the same way but is run in the Makefile too, so
the TAG variable applied here too, but not the platform.
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Co-authored-by: Paul McKeown <paul@midships.io>
e64c4b8ab1