Image cache lookup should support SHA256 digests and Image IDs

See #1391 (and this may be related to #699):

Testcontainers' image indexing code can't handle images that have SHA256 digests. We need a way to access the RepoDigests from docker-java's Image class. Unless I'm missing something, at the moment only RepoTags is mapped.

We would then modify RemoteDockerImage:

                List<Image> updatedImages = listImagesCmd.exec();
                updatedImages.stream()
                    .map(Image::getRepoTags)
                    .filter(Objects::nonNull)
                    .flatMap(Stream::of)
                    .map(DockerImageName::new)
                    .collect(Collectors.toCollection(() -> AVAILABLE_IMAGE_NAME_CACHE));

so that both Image::getRepoTags and Image::getRepoDigests are loaded into the cache. There's probably also some work to do around identity/equals/hashcode of DockerImageName objects in the cache.

EDIT 5 June 2020: We should also ensure that Image IDs are loaded into the cache - see #699.

Start with RemoteDockerImage's listImagesCmd cache population and inspect docker-java's Image mapping for RepoTags, RepoDigests, and the image ID. Then review DockerImageName identity, equals, and hashCode behavior in the cache. Done means repository tags, SHA256 digests, and image IDs can all be used for image lookup, with appropriate tests added or updated.