skopeo will disable the progress bar if it detects that stdout isn't a
TTY - in order to make it think that stdout _isn't_ a TTY and therefore
avoid it printing a lot of "…" on separate lines, we pipe the output
through cat.
This changes the output from:
…
…
…
…
…
…
to the eminently more useful and less spammy:
Getting image source signatures
Copying blob sha256:[snip]
Copying blob sha256:[snip]
Copying blob sha256:[snip]
Copying config sha256:[snip]
Writing manifest to image destination
Storing signatures
48c23a086b