Support DOCKER_REGISTRY_URL env var

[msabramo]

If the environment variable DOCKER_REGISTRY_URL is set, then the value of it will be used as the default registry URL, instead of "https://index.docker.io".

E.g.:

$ export DOCKER_REGISTRY_URL=https://docker.corp.surveymonkey.com
$ docker-ls repositories
requesting list from https://docker.corp.surveymonkey.com . done
repositories:
- acme/webhook
- admintools/admintools-nodejs-asset-build
- cx
...

[zoobab]

Any idea when this is gonna merged/refused? This PR is already more then one month old...

[DirtyHairy]

Hi @msabramo !

Sorry for taking so long to respond to your PR. The code looks mostly fine to me; however, there's some error handling missing in initRegistryUrl. Specifically, you don't test for parse errors during url.Parse, leading to a potentially uninitialized url struct. I would prefer to change DEFAULT_REGISTRY_URL and Config.registryUrl to pointers that are set to nil in this case. This way, the client can check whether it has a valid (non-nil) url at startup and throw an error in the vein of "no valid URL configured" otherwise.

Would you mind making these changes?

Cheers
-Christian

[zoobab]

This is the most useful feature for me.

[DirtyHairy]

v0.3.0 supports configuring the registry via DOCKER_LS_REGISTRY. Still, thanks for your work @msabramo