chore: remove ARM support

Building/testing for ARM does not happen often - the only runtime environment is Raspberry PI, which supports ARM64 - and complicates support for a ton of features. Aside from that, CI runs are much longer for ARM Dokku images, often reaching 15-20 minutes or just timing out completely.

Rather than support an architecture that doesn't have much usage by maintainers and has a lot of maintenance burden, we're removing the platform.

.github/actions/build-image/action.yml

@@ -25,15 +25,7 @@ runs:
     - name: build docker image
       shell: bash
       run: |
-        if [[ "${{ inputs.architecture }}" == "linux/arm" ]]; then
-          docker buildx build \
-                --platform linux/arm \
+        docker buildx build \
                 --progress plain \
                 --tag "dokku/dokku:latest" \
                 .
-        else
-          docker buildx build \
-                  --progress plain \
-                  --tag "dokku/dokku:latest" \
-                  .
-        fi