Publish Skupper source as OCI image (skupper-source) for reproducible downstream builds

[AryanP123]

Add a minimal Containerfile and release step to publish the Skupper source tree to Quay as quay.io/skupper/skupper-source: (e.g. 2.2.1). Downstream container builds can then use this via build context instead of git clone at build time. Related to IBM Delivery Platform / CSD onboarding; separate follow-up will update container-*-ibm Dockerfiles.