Every umoci operation that modifies an image will not delete any now-unused
blobs in the image (so as to ensure that any other operations that assume those
blobs are present will not error out). However, this will result in a large
number of useless blobs remaining in the image after operating on an image for
a long enough period of time. umoci gc will garbage collect all blobs that
are not reachable from any known image tag. Note that calling umoci gc
between umoci unpack and umoci repack may result in errors if you’ve
removed all references to the blobs used by umoci unpack.
% umoci gc --layout opensuse