Update trigger, inputs according to the review

.github/workflows/docker-push.yml

@@ -1,11 +1,12 @@
 ---
 name: 'Push Docker Image'
 on:
-  # TODO: remove, only for testing; replace with
-  # workflow_dispatch:
-  push:
-    branches:
-      - '**' # Trigger on every branch
+  workflow_dispatch:
+
+    inputs:
+      kontrol_branch:
+        description: "Branch of Kontrol to use to build the Docker image"
+        required: true
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -19,14 +20,14 @@ jobs:
       - name: 'Check out code'
         uses: actions/checkout@v4
         with:
-          ref: ${{ github.event.push.head.sha }}
+          ref: ${{ github.events.inputs.kontrol_branch}}
           fetch-depth: 0
 
       - name: 'Set environment'
         run: |
           KONTROL_VERSION=$(cat package/version)
           echo "CONTAINER_NAME=kontrol-ci-docker-${GITHUB_SHA}" >> ${GITHUB_ENV}
-          BRANCH_NAME="${{ github.ref_name }}"
+          BRANCH_NAME="${{ github.events.inputs.kontrol_branch }}"
           SANITIZED_BRANCH_NAME=$(echo "${BRANCH_NAME}" | tr '/' '-' | tr -cd '[:alnum:]-_.')
           GHCR_TAG=ghcr.io/runtimeverification/kontrol/kontrol:ubuntu-jammy-${SANITIZED_BRANCH_NAME}
           echo "GHCR_TAG=${GHCR_TAG}" >> ${GITHUB_ENV}