Add docker.io image push

With this commit we publish on ghcr.io, quay.io and docker.io.

Fixes: #15

.github/workflows/main.yml

@@ -82,6 +82,14 @@ jobs:
         username: ${{ vars.QUAY_ROBOT_NAME }}
         password: ${{ secrets.QUAY_ROBOT_TOKEN }}
 
+    - name: Login to docker.io
+      if: github.event_name != 'pull_request'
+      uses: docker/login-action@v3
+      with:
+        registry: docker.io
+        username: ${{ vars.DOCKER_USER }}
+        password: ${{ secrets.DOCKER_TOKEN }}
+
     - name: Build and push
       uses: docker/build-push-action@v5
       with:
@@ -95,3 +103,5 @@ jobs:
            ghcr.io/mmul-it/kpa:${{ github.ref_name }}
            quay.io/mmul/kpa:latest
            quay.io/mmul/kpa:${{ github.ref_name }}
+           docker.io/mmul/kpa:latest
+           docker.io/mmul/kpa:${{ github.ref_name }}

README.md

@@ -21,8 +21,10 @@ be no need to rely on proprietary tools and formats to create contents.
 **KPA** is at the same time:
 
 - The definition of an approach, described in this page.
-- A container named **kpa** available in the
-  [GitHub Container Registry](https://ghcr.io/mmul-it/kpa).
+- A container named **kpa** available on the main public container registries:
+  - [GitHub Container Registry](https://ghcr.io/mmul-it/kpa).
+  - [Quay](https://quay.io/repository/mmul/kpa).
+  - [Docker Hub](https://hub.docker.com/r/mmul/kpa).
 - An Ansible role named **kpa_generator** available in
   [Ansible Galaxy](https://galaxy.ansible.com/mmul/kpa_generator).
 
@@ -297,7 +299,7 @@ Your command line will be something like this:
 > docker run --rm \
   -v $PWD:/kpa/projects/example \
   -v /tmp:/kpa/output \
-  quay.io/mmul/kpa --project example --yml Example-Training-01.yml
+  ghcr.io/mmul/kpa --project example --yml Example-Training-01.yml
 ```
 
 After a really short time you should get: