dockerTools.pullImage: force disable skopeo's progress bar

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

pkgs/build-support/docker/default.nix

@@ -117,7 +117,8 @@ rec {
         --tmpdir=$TMPDIR \
         --override-os ${os} \
         --override-arch ${arch} \
-        copy "$sourceURL" "docker-archive://$out:$destNameTag"
+        copy "$sourceURL" "docker-archive://$out:$destNameTag" \
+        | cat  # pipe through cat to force-disable progress bar
     '';
 
   # We need to sum layer.tar, not a directory, hence tarsum instead of nix-hash.