attach: don't return context cancelled error

[laurazard]

closes#5294

- What I did

In #4993 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 how we use this context in docker attach, this caused us to start returning a 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.

- How to verify it

Run the added test:

TESTDIRS="./e2e/container/..." TESTFLAGS="-test.run=TestAttachInterrupt" make -f docker.Makefile test-e2e-non-experimental

- Description for the changelog

Fix `docker attach` printing a spurious `context cancelled` error message.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.46%. Comparing base (1e0f669) to head (66aa0f6).
Report is 2 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #5295 +/- ##
==========================================
+ Coverage 60.99% 61.46% +0.46% 
==========================================
Files 296 299 +3 Lines 20846 20856 +10 ==========================================
+ Hits 12716 12820 +104 + Misses 7212 7120 -92 + Partials 918 916 -2 

[thaJeztah]

FWIW, I recall we also added a utility to errdefs, but not sure if we also must ignore the other context error for these

cli/vendor/github.com/docker/docker/errdefs/is.go

Line 121 in 0cc1b8c

funcIsContext(errerror) bool {

[thaJeztah]

LGTM, thanks!