Bug 1490964 - tighten docker-worker payload schema

[petemoore]

This tightens the screws on the payload schema of docker-worker primarily by adding additionalProperties settings throughout the JSON subschemas.

This is useful for two reasons:

1. Currently additional properties can be submitted without causing a malformed payload exception, that would be ignored by docker-worker (e.g. misspelled properties) and would cause hard-to-diagnose failures for task submitters
2. It causes cleaner types to be generated by github.com/taskcluster/jsonschema2go which will simplify and improve the process of adding support for docker-worker payloads in generic-worker

See bug 1490964 for further context.