Only reset compression if its not valid

If we reset the default compression method, force compression no longer works.

Signed-off-by: James Hewitt <[email protected]>

libimage/push.go

@@ -86,8 +86,16 @@ func (r *Runtime) Push(ctx context.Context, source, destination string, options
 		destRef = dockerRef
 	}
 
-	// docker-archive and only DockerV2Schema2MediaType support Gzip compression
-	if options.CompressionFormat != nil &&
+	// docker-archive and DockerV2Schema2MediaType support only Gzip compression
+	// If the CompressionFormat has come from containers.conf (set as a default),
+	// but isn't supported for this push, we want to ignore it.
+	// If the CompressionFormat has come from the CLI (ForceCompressionFormat
+	// requires CompressionFormat to be set), we want to strip the invalid value
+	// so that the push attempt fails.
+	//
+	// Ideally this should all happen at a much higher layer, where the code can differentiate
+	// between a value coming from containers.conf vs. the CLI.
+	if options.CompressionFormat != nil && options.CompressionFormat.Name() != "gzip" &&
 		(destRef.Transport().Name() == dockerArchiveTransport.Transport.Name() ||
 			destRef.Transport().Name() == dockerDaemonTransport.Transport.Name() ||
 			options.ManifestMIMEType == manifest.DockerV2Schema2MediaType) {