Fix e2e test by using a pinned version of alpine

[dnephin]

Fixes this test failure on master, caused by a repin of alpine:3.6. This is now using a digest to pull a specific version of alpine so we shouldn't run in to this problem again.

22:44:11 === RUN   TestRunAttachedFromRemoteImageAndRemove
22:44:11 --- FAIL: TestRunAttachedFromRemoteImageAndRemove (0.72s)
22:44:11 		
	            	-Digest: sha256:0930dd4cc97ed5771ebe9be9caf3e8dc5341e0b5e32e8fb143394d7dfdfa100e
22:44:11 		
	            	+Digest: sha256:641b95ddb2ea9dc2af1a0113b6b348ebc20872ba615204fbe12148e98fd6f23d
22:44:11 		
	            	 Status: Downloaded newer image for registry:5000/alpine:test-run-pulls
22:44:11 		
22:44:11 FAIL
22:44:11 FAIL	github.com/docker/cli/e2e/container	0.721s

[codecov-io]

Codecov Report

Merging #532 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #532   +/-   ##
=======================================
  Coverage   49.05%   49.05%           
=======================================
  Files         200      200           
  Lines       16398    16398           
=======================================
  Hits         8044     8044           
  Misses       7935     7935           
  Partials      419      419

[vdemeester]

LGTM 🐮

[thaJeztah]

LGTM