Allow to set tag in workflow dispatch.

.github/workflows/build-env.yml

@@ -2,13 +2,21 @@ name: Update build environment
 
 on:
   workflow_dispatch:      # Trigger by hand from the UI
+    input:
+      tag:
+        description: 'tag for docker image'
+        required: true
+        default: 'latest'
+        type: string
   schedule:
     - cron: '0 0 * * 0'   # Schedule it every Sunday
 
 jobs:
   build-spack-pyprecice-deps:
     name: Builds the baseimage for spack providing dependencies
     runs-on: ubuntu-latest
+    env:
+      TAG: ${{ inputs.tag }}
     steps:
       - name: Checkout Repository
         uses: actions/checkout@v2
@@ -25,4 +33,4 @@ jobs:
           push: true
           context: .
           file: spack/ci-spack-pyprecice-deps-2404.dockerfile
-          tags: precice/ci-spack-pyprecice-deps-2404
+          tags: "precice/ci-spack-pyprecice-deps-2404:$TAG"