Add support for OCI spec annotations (and legacy LABEL)

[pombredanne]

These can be added as LABEL from a Dockerfile https://github.com/Tekki/docker-perl-test/blob/e52c397f9daf04ca89c416b76b610da5ae1cc15a/p16/Dockerfile#L24 and were initially specified in http://label-schema.org/rc1/ and are now specified at https://github.com/opencontainers/image-spec/blob/master/annotations.md

Some key/values are predefined:
https://github.com/opencontainers/image-spec/blob/master/annotations.md#pre-defined-annotation-keys :

• org.opencontainers.image.created: date/timestamp
• org.opencontainers.image.authors: contact details of the people or organization responsible for the image (freeform string)
• org.opencontainers.image.url: homepage URL
• org.opencontainers.image.documentation: documentation URL
• org.opencontainers.image.source: VCS/download URL
• org.opencontainers.image.version: version ideally semver and mapped to a VCS repo tag
• org.opencontainers.image.revision: VCS revision e.g. git commit in org.opencontainers.image.source
• org.opencontainers.image.vendor: author (e.g. created_by)
• org.opencontainers.image.licenses: Should be an SPDX License Expression
• org.opencontainers.image.ref.name: unclear... "Name of the reference for a target"
• org.opencontainers.image.title: brief summary or "name" of the image?
• org.opencontainers.image.description: long description

These are some examples:

• json.txt
• config.json.zip