This method looked slightly incomplete. If the port wasn't found,
it'd return `err`, but that was always `nil`, so we'd print out
`:0`.
Now, we construct a nice error message with the targeted port and
the ones we found.
The `--protocol` flag is also now case-insensitive to prevent any
weirdness/confusion there.
Co-authored-by: Nick Sieger <nicksieger@gmail.com>
Signed-off-by: Milas Bowman <milas.bowman@docker.com>
Nicolas De Loof
Milas Bowman
Guillaume Lours
Nick Sieger