Minor updates to docker Makefile and scan job

[ashahba]

This PR:

• updates Makefile and Trivy workspace file so that the docker build always pulls latest base ubuntu:22.04 first
• updates Makefile so that the container is saved in gzip format rather than tar so the saved image has a smaller footprint:

$ du -sh openfl.tar*
1.2G 	openfl.tar
463M	openfl.tar.gz

• Removes stock ssh keys installed under /etc/ssh/ by openssh server, since they are not needed and also Trivy flags them incorrectly as secrets left on the container.

[psfoley]

Thank you @ashahba! Approved