In 3f0d90a2a9 we introduced a global
signal handler and made sure all the contexts passed into command
execution get (appropriately) cancelled when we get a SIGINT.
Due to that change, and how we use this context during `docker attach`,
we started to return the context cancelation error when a user signals
the running `docker attach`.
Since this is the intended behavior, we shouldn't return an error, so
this commit adds checks to ignore this specific error in this case.
Also adds a regression test.
Signed-off-by: Laura Brehm <laurabrehm@hey.com>
66aa0f672c