docker: Use Ubuntu ports repositories on non-x86 architectures

docker: Use Ubuntu ports repositories on non-x86 architectures

This should allow Patchwork to run "out of the box" in Docker on any
architecture with a) an Ubuntu port and b) support in the Postgres
multiarch Docker image, which includes at least arm64 and ppc64le.

It's a little gross hacking the Dockerfile like this, but I'm not sure
there's a more elegant way to do it.  Unfortunately it doesn't seem like
there's any way to do conditional COPY, and anything in RUN is plain
/bin/sh, so that's why it looks like it does.

Tested on ppc64le and on x86_64.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>