How to shorten docker build-push-action image name

The name of a docker image tag needs to include a registry hostname when you use docker push (unless you are pushing to the default Docker’s public registry located at registry-1.docker.io)

It is how docker push knows where to push to.

That is why a typical "build and push" step looks like:

name: Build and Push Docker container

on:
  push:
    branches:
      - main

jobs:
  build-and-push:

    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v1

    - name: Build the Docker image
      run: docker build -t ghcr.io/<<ACCOUNT NAME>>/<<IMAGE NAME>>:<<VERSION>> .

    - name: Setup GitHub Container Registry
      run: echo "$" | docker login https://ghcr.io -u $ --password-stdin

    - name: push to GitHub Container Registry
      run:  docker push ghcr.io/<<ACCOUNT NAME>>/<<IMAGE NAME>>:<<VERSION>>

In your case, (using the docker/build-push-action), even if you were to use a local registry, you would still need to tag accordingly.

-
        name: Build and push to local registry
        uses: docker/build-push-action@v3
        with:
          context: .
          push: true
          tags: localhost:5000/name/app:latest
                ^^^^^^^^^^^^^^
                # still a long tag name