Treat Dockerfile and Containerfile as "non-assembled" package data, collect OCi labels

[pombredanne]

It would useful as these are package like and have structured data as seen in https://github.com/jitsi/docker-jitsi-meet/blob/stable-8960-1/jibri/Dockerfile#L5

LABEL org.opencontainers.image.title="Jitsi Broadcasting Infrastructure (jibri)"
LABEL org.opencontainers.image.description="Components for recording and/or streaming a conference."
LABEL org.opencontainers.image.url="https://github.com/jitsi/jibri"
LABEL org.opencontainers.image.source="https://github.com/jitsi/docker-jitsi-meet"
LABEL org.opencontainers.image.documentation="https://jitsi.github.io/handbook/"

See for full spec: https://github.com/opencontainers/image-spec/blob/main/annotations.md

Note that container-inspector already supports parsing Dockerfile.