Move to GHCR for builds (test build)

Changelog for 0.12.15:

• PR #839 Migrate builds from Travis to Github Actions by @utsavanand2
• PR #840 Allow configuration of CLI config via env variables by @LucasRoesler
• PR #837 Update alpha namespaces to v1 by @alexellis
• PR #835 Fix #829 to fix Knative spec generation by @alexellis

Commits
15cc71a Migrate builds from Travis to Github Actions by @utsavanand2
d8c8e34 Allow configuration of CLI config via env variables by @LucasRoesler
6914d95 Update alpha namespaces to v1 by @alexellis
03738d2 Remove quay.io by @alexellis
6401791 Update knative help message by @alexellis
c8fbda1 Migrate to GA Knative serving service by @alexellis
f04894c Fix #829 to fix Knative spec generation by @alexellis

Generated by Derek