feature : replace id & repo with variables

to avoid possible typo

saw a prior case

Author: beachdweller

.github/workflows/docker-publish.yml

@@ -15,8 +15,8 @@ jobs:
       - name: Prepare
         id: prep
         run: |
-          DOCKERHUB_IMAGE=kangwonlee/gemini-python-tutor
-          GHCR_IMAGE=ghcr.io/${{ github.repository_owner }}/gemini-python-tutor
+          DOCKERHUB_IMAGE=${{ github.repository_owner }}/${{ github.repository }}
+          GHCR_IMAGE=ghcr.io/${{ github.repository_owner }}/${{ github.repository }}
           VERSION=${GITHUB_REF#refs/tags/v}
           MINOR=${VERSION%.*}
           MAJOR=${VERSION%%.*}