Create DOCKER_USER variable for developer to build and push docker image for test

[JohnNiang]

What this PR dose?

Create DOCKER_USER variable for developer to build and push docker image for test

Why we need it?

Before this PR, if we want to build and push a docker image for test, we may do with the following steps:

Approach 1

1. Change Makefile and modify surenpai/devops-controller:$(VERSION)-$(COMMIT) to johnniang/devops-controller:$(VERSION)-$(COMMIT)
2. Execute command: make docker-build-push-controller

Approach 2

1. Execute make docker-build-push-controller
2. Execute docker tag surenpi/devops-controller:dev-v3.2.0-alpha.1-7244edd johnniang/devops-controller:dev-v3.2.0-alpha.1-7244edd
3. Execute docker push johnniang/devops-controller:dev-v3.2.0-alpha.1-7244edd

But now, we just need to execute DOCKER_USER=johnniang make docker-build-push-controller to archive task above.

/kind feature

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[codecov]

Codecov Report

Merging #302 (978ee91) into master (6c0564c) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #302      +/-   ##
=========================================
- Coverage    7.64%   7.61%   -0.03%     
=========================================
  Files          90      90              
  Lines       21357   21357              
=========================================
- Hits         1632    1627       -5     
- Misses      19602   19605       +3     
- Partials      123     125       +2     

Flag	Coverage Δ
unittests	7.61% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
controllers/pipeline/pipeline_controller.go	38.23% <0.00%> (-3.68%)	⬇️
controllers/s2irun/s2irun_controller.go	33.57% <0.00%> (-3.65%)	⬇️
...rs/devopscredential/devopscredential_controller.go	36.48% <0.00%> (+3.37%)	⬆️

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[LinuxSuRen]

I prefer changing this to kubespheredev

[LinuxSuRen]

For me, surenpi is my personal account. But kubespheredev is not a user account. So, I suggest replacing DOCKER_USER to be a better name.

[JohnNiang]

Due to LinuxSuRen/open-source-best-practice#33, I'm going to close this PR and let this chance leave to new contributors.

/close