test rmi -f returns success

[andrewhsu]

- What I did

Added e2e test to verify docker rmi -f returns unix code success always because -f.

- How I did it

At my desk.

- How to verify it

$ make -f docker.Makefile test-e2e
...
=== RUN   TestForceRemoveImage
--- PASS: TestForceRemoveImage (0.16s)
PASS

- Description for the changelog

N/A

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

🐛

[codecov-io]

Codecov Report

Merging #567 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
+ Coverage    49.4%   49.41%   +0.01%     
==========================================
  Files         208      208              
  Lines       17159    17159              
==========================================
+ Hits         8477     8479       +2     
+ Misses       8249     8247       -2     
  Partials      433      433

[dnephin]

We already have test coverage for that here in the unit test suite: https://github.com/docker/cli/blob/master/cli/command/image/remove_test.go#L71-L78

Following https://github.com/docker/cli/blob/master/TESTING.md#guidelines I don't think we need an e2e test for the force flag.

I unit test for a successful docker rmi would be good, but it would go in the e2e/image package.