Proposal: Add support for custom base image repository in the Dockerfile

Description of the problem
I have been encountering rate limit errors when pulling images from Docker Hub. To address this, I'm using a Harbor project as a cache. It would be helpful to modify the Dockerfile in the GitHub Action to allow customizing the base image repository.

Proposal
Modify the Dockerfile to allow the use of a custom repository for the FROM statement, like this:

ARG REPOSITORY FROM ${REPOSITORY}/dokku/ci-docker-image:0.13.0

Add a new optional parameter, REPOSITORY, to the GitHub Action.

• If the REPOSITORY variable is provided, it should be used in the FROM statement.
• If it’s empty, it should default to:

FROM dokku/ci-docker-image:0.13.0

Also add the credential configuration to access, in my case project cache in private harbor. As a temporary solution I have created a public project and with that I do not have an authentication error:

Expected Results

• Users can configure the REPOSITORY in the GitHub Action inputs.
• If the REPOSITORY is not set, the action should default to using dokku/ci-docker-image:0.13.0 from Docker Hub.

@josegonzalez i from Chile,

Regards
Saludos

Start by locating the Dockerfile used by the GitHub Action and the action input/build entry point; no file paths or tests are named in the issue. Check how the current base image is selected and how registry credentials are configured. Done means an optional REPOSITORY input changes the base image, preserves the Docker Hub default when empty, and supports the requested private-registry authentication.