floss: Replace docker with podman

Docker requires root permissions to run and is not preferable for
security reasons. Podman is a drop-in replacement that doesn't run as
root.

Bug: 220203992
Tag: #floss
Test: ./docker-build-image.py --tag floss:latest; ./build-in-docker.py
Change-Id: I813fb63a25bf5588560090ddfb0ad290729a4ff1