Make Docker URL for fetching Templates flexible

[tarunwadhwa-paypay]

Is your feature request related to a problem? Please describe.

My company uses a proxy for fetching docker images (which allows caching if needed). The endpoint for listing tags is different from docker hub endpoint. But since /tags is hardcoded here, I cannot simply set the DOCKER_HUB_URL value to use my custom proxy

Describe the solution you'd like

I will be open to raising a PR if you feel this can be added.
Proposed solution -
Create a template URL in env config. Something like -
- DOCKER_HUB_TEMPLATE_URL=https://hub.docker.com/v2/repositories/{repo_name}/tags
and code can simply format the repo_name in code

Although the original variable (DOCKER_HUB_URL) is not being used anywhere, hardcoding /tags in value might not keep code extensible, hence new variable altogether

Describe alternatives you've considered
Nothing possible as such since /tags will give 404 for our proxy