Resolve add linux as cross-compilation platform

[temrix]

This resolves #28 by doing the following:

• Remove unnecessary dependencies from the cross-Dockerfile for Windows as they are already present in the upstream image.
• Add .dockerignore so that /target is not sent to the build context.
• Enable the software collections in linux-cross. Reference: https://austindewey.com/2019/03/26/enabling-software-collections-binaries-on-a-docker-image/
• The FROM statements have fully-qualified names. Important for podman-support.