[change] Make image owner configurable in docker-compose

[pandafy]

Is your feature request related to a problem? Please describe.
Currently, the user/owner for the Docker image is hardcoded as openwisp in the docker-compose.yml file and set as a variable in the Makefile. This makes it difficult to customize ownership for deployments that have different requirements or need to pull from a private registry/custom namespace.

For example, in Makefile:

USER = registry.gitlab.com/openwisp/docker-openwisp

And in docker-compose.yml:

image: openwisp/openwisp-dashboard:latest

Describe the solution you'd like
Allow the image owner to be configured via an environment variable (e.g., IMAGE_OWNER) in the .env file and used in docker-compose.yml. The solution should also unify this with the Makefile to ensure consistency.

Example implementation in docker-compose.yml:

image: ${IMAGE_OWNER:-openwisp}/openwisp-dashboard:${OPENWISP_VERSION:-latest}

P.S.: We can use a better variable name than IMAGE_OWNER, maybe just use USER.

Additional context
See related discussion: PR 555 discussion.

[Srinath0916]

Hey @pandafy , I'd like to take this one. I'll make the image owner configurable across docker-compose.yml, .env, and Makefile using IMAGE_OWNER with openwisp as the default. Let me know if you want me to go ahead.