Bump Golang 1.12.1

[thaJeztah]

Counterpart for the engine is in moby/moby#38404

Making this "WIP" for now, as there's a known regression in Go 1.12.0 that's affecting the engine, so let's wait for 1.12.1 moby/moby#38404 (comment)

[thaJeztah]

Ah, caught some failures;

=== Errors
cli/config/config_test.go:154:3: Fatalf format %q has arg config of wrong type *github.com/docker/cli/cli/config/configfile.ConfigFile
cli/config/config_test.go:217:3: Fatalf format %q has arg config of wrong type *github.com/docker/cli/cli/config/configfile.ConfigFile
cli/config/config_test.go:253:3: Fatalf format %q has arg config of wrong type *github.com/docker/cli/cli/config/configfile.ConfigFile
cli/config/config_test.go:288:3: Fatalf format %q has arg config of wrong type *github.com/docker/cli/cli/config/configfile.ConfigFile
cli/config/config_test.go:435:3: Fatalf format %q has arg config of wrong type *github.com/docker/cli/cli/config/configfile.ConfigFile
cli/config/config_test.go:448:3: Fatalf format %q has arg config of wrong type *github.com/docker/cli/cli/config/configfile.ConfigFile

DONE 1115 tests, 2 skipped, 6 errors in 215.984s
make: *** [Makefile:22: test-coverage] Error 2
Exited with code 2

[thaJeztah]

Ahm.. more to fix; https://jenkins.dockerproject.org/job/docker/job/cli/job/PR-1708/2/execution/node/27/log/?consoleFull

17:45:30 + E2E_UNIQUE_ID=clie2e2 IMAGE_TAG=clie2e2 make -f docker.Makefile test-e2e
17:45:30 docker build -t docker-cli-e2eclie2e2 --build-arg VERSION=19.03.0-dev --build-arg GITCOMMIT= -f ./dockerfiles/Dockerfile.e2e .

...

17:48:22 === Errors
17:48:22 build cache is required, but could not be located: GOCACHE is not defined and neither $XDG_CACHE_HOME nor $HOME are defined
17:48:22 

[codecov-io]

Codecov Report

Merging #1708 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1708   +/-   ##
=======================================
  Coverage   56.23%   56.23%           
=======================================
  Files         307      307           
  Lines       21125    21125           
=======================================
  Hits        11880    11880           
  Misses       8382     8382           
  Partials      863      863

[thaJeztah]

created separate PR's for the fixes;

• Fix test for Go 1.12.x #1712 Fix test for Go 1.12.x
• Fix: plugin-tests discarding current environment #1713 Fix: plugin-tests discarding current environment

[thaJeztah]

rebased this on top of #1742, so that we can merge that one first (and cherry-pick into the 18.09 release branch)

[thaJeztah]

rebased; this should be ready to go now 👍

[silvin-lubecki]

LGTM