Add support for content trust

[ghost]

Docker content trust can be automatically enabled by setting the following env vars: DOCKER_CONTENT_TRUST, DOCKER_CONTENT_TRUST_ROOT_PASSPHRASE, DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE.

Settings these variables triggers the creation of signing keys but they need to persisted for reuse. This should be implemented in the plugin.

[tboerger]

Maybe we could add env variables to inject the keys. Storing anything generated by the plugin will be out of the scope of this plugin.