Implement ImageManager trait for docker use

- Changed docker.rs to contain the implementation of ImageManager trait.
  The DockerImageManager struct uses the old logic with shiplift.
- Changed lib.rs to use a runtime-set image manager, which can be oci or
  docker, depending if the new --oci-uri flag is used or not.
- Refactored the 'inspect_image' method to return OCI-compliant image
  metadata in case OciImageManager is used, instead of a Docker-specific one.
- Changed enclave_build/src/main.rs to include the new additions.
- Matched the OCI behavior by pulling or building the Docker image on
  manager creation based on the present arguments.

Signed-off-by: Calin-Alexandru Coman <[email protected]>
Signed-off-by: Raul-Ovidiu Moldovan <[email protected]>