Allow custom registry/proxy

[akhayyat]

Problem: In an environment where direct Internet access is not available and a local docker registry proxy is used, all docker hub images are available as docker.my.tld/<docker-hub-name>.
Consequently, the image docker.io/firecow/gitlab-ci-local-util:latest cannot be pulled.

Suggested solution: Provide means to configure the docker registry URL (replacement for docker.io), e.g. a DOCKER_REGISTRY configuration variable.

In the meantime, one possible alternative is to configure the Docker daemon to use a registry mirror as documented here:
https://docs.docker.com/docker-hub/mirror/#configure-the-docker-daemon

[ANGkeith]

In the meantime, one possible alternative is to configure the Docker daemon to use a registry mirror as documented here:
https://docs.docker.com/docker-hub/mirror/#configure-the-docker-daemon

In my opinion, your proposed alternative is sufficient.

I don’t see the need to implement this in gitlab-ci-local, since docker is unlikely to function properly without configuring the docker daemon. You would eventually need to update the configuration there as well, so might as well handle it directly at that level, effectively solving two problems with a single configuration.