Support third-party registries

[StephenWeatherford]

Specifically requested (but goal is generic support), in no particular order:

Open:

• goharbor.io (see harbor login #878, connect registries error #3449, container registries can't login to repo1.dso.mil even tho it supports Docker HTTP API V2 #3965)
• ECR (Amazon, see Error connecting to ECR with temporary password #3202, AWS ECR docs #3604)
• GCR (Google) (+3 see Google container registry? #394, Add Google Cloud Products in list to Connect Registry. #2890)
• quay.io (+4 see Add quay.io as registry option #390)
• DTR (Enterprise Docker trusted registry) (see Can't sign in to Docker Enterprise Trusted Registry 2.7.8 #3325)
• Cloudsmith (see Adding Cloudsmith Registry #1887)
• Aliyun (see Not Support Aliyun Docker registry #1964)
• Portus (see Can't sign-in to Generic Docker Registries #2735)
• Digital Ocean (see Support for Digital Ocean registry #2879)
• Generic registries with self-signed certificate (docker register: failed,reason:self signed certificate #3639)
• Artifactory (Unable to connect to jfrog.io-based Artifactory Cloud registry using Docker extension #4011)

Blocked on no/partial Registry V2 support:

• V2 registries that don't use /v2/ endpoint (see The extension corrupts the URL to the docker registry, making it impossible to connect. #3562)
• GitLab self-hosted (+6 see Allow to use self hosted gitlab registry. #1832, Support for self-managed gitlab instance #2457, Can't sign-in to Generic Docker Registries #2735, Support for self-hosted gitlab  #3119, Can't connect to custom Gitlab instance in docker registry #3271, Configuration Option Self-hosted GitLab Instances #3813)
• harbor.korporate.org (see registry conected but eror 405 #1927)
• GitLab Groups (see Add GitLab Groups to list of supported registry providers #1970)
• IBM Container Registry (see Unable to connect to IBM Container Registry #3961)

Done:

• DockerHub
• Azure
• GitLab (+25-ish see add access to gitlab.com registry #162)
• Generic V2 registry (+2)
• GitHub (see Add Github Registry to the supported list #1525, Add support for GitHub pacakges registry #1938, Connect to GitHub #3686)

[gauravlakhani]

while connecting using private registry option, is private Docker DTR (Enterprise Docker trusted registry) supported at all?

[StephenWeatherford]

@gauravlakhani Support for registries beyond Docker Hub is fairly new, we haven't tested on DTR. I've added it to the list.

[shaneholloman]

@here champion!