[change] Make image owner configurable in docker-compose

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.

Start by comparing the image definitions in docker-compose.yml and the Makefile, then read the related PR 555 discussion for naming and consistency concerns. Verify the resulting Compose configuration with the relevant environment values and confirm that the image owner can be changed in one place while the default behavior remains unchanged.