Add Docker Login Command for ACR

[listonb]

• Execute a docker login for a given container registry

[msftclas]

@listonb,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[codecov-io]

Codecov Report

Merging #3734 into master will decrease coverage by 0.02%.
The diff coverage is 23.07%.

@@            Coverage Diff             @@
##           master    #3734      +/-   ##
==========================================
- Coverage   72.09%   72.07%   -0.03%     
==========================================
  Files         422      422              
  Lines       26259    26272      +13     
  Branches     3995     3996       +1     
==========================================
+ Hits        18932    18935       +3     
- Misses       6099     6106       +7     
- Partials     1228     1231       +3

Impacted Files	Coverage Δ
...-cli-acr/azure/cli/command_modules/acr/commands.py	100% <100%> (ø)	⬆️
...li-acr/azure/cli/command_modules/acr/credential.py	51.61% <16.66%> (-22.08%)	⬇️
...nent/azure/cli/command_modules/component/custom.py	16.23% <0%> (ø)	⬆️
src/azure-cli-core/azure/cli/core/util.py	70.06% <0%> (ø)	⬆️
...dback/azure/cli/command_modules/feedback/custom.py	31.25% <0%> (ø)	⬆️

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[djyou]

@listonb Thank you for the PR. We planned a PR to be out in the next a few days with our latest support for managed registries, webhooks, AAD-based docker login with az acr login (so that regardless whether admin user is enabled we will log you in) and more repository operations (including delete). For the docker login command specifically, you can take a look at our work in progress here https://github.com/AppPlatPerf/azure-cli/blob/master/src/command_modules/azure-cli-acr/azure/cli/command_modules/acr/custom.py#L213 Your PR is a subset of what we are preparing except that we definitely want to have an az acr login command rather than az acr credential login. I will cc you once our PR is out. /cc @sajayantony @DavidObando

[listonb]

@djyou Great. My only addition would be to verify docker is installed which the PR you referred to currently does not. I will close this PR.