streamline dockerfile and GHA docker publishing

[mchangrh]

• no longer split GHCR and docker.io registries
• update actions to latest available versions
• add ARM images to dockerhub
• remove redundant WORKDIR commands
• apk add --no-cache to reduce file size

[marcopeocchi]

LGTM! Thanks! 🤠

[mchangrh]

Oops, looks like you use a different username for dockerhub, I'll PR another fix for that

[marcopeocchi]

No worries!