Patch:ci:Allow the maptool image to be built for any branch

container.yml builds the maptool container image only for a push to trunk, and
it always uses the tag "trunk". A branch under review therefore has no image,
and tools that consume the image cannot test that branch.
gh-actions-mapserver has this problem: it converts the maps with
ghcr.io/navit-gps/navit/maptool:trunk.

This change adds the workflow_dispatch trigger and takes the tag from the
branch name. A push to trunk still produces the tag "trunk", so nothing
changes for the existing image.

The branch name reaches the shell through the environment, and not through the
command line, because a branch name is untrusted input. A slash in a branch
name becomes a hyphen, because a tag cannot contain a slash.

Author: pgrandin

.github/workflows/container.yml

@@ -4,6 +4,10 @@ on:
     push:
         branches:
             - trunk
+    # Lets you build the image for any other branch, so that a branch under
+    # review can be tested with the tools that use this image. The tag is the
+    # name of the branch.
+    workflow_dispatch:
 
 jobs:
     build:
@@ -13,6 +17,15 @@ jobs:
             - name: Checkout code
               uses: actions/checkout@v7
 
+            - name: Get the image tag from the branch name
+              id: image
+              # A branch name can contain a slash, which a tag cannot.
+              # github.ref_name goes through the environment, because a branch
+              # name must not become part of the command.
+              env:
+                REF_NAME: ${{ github.ref_name }}
+              run: echo "tag=${REF_NAME//\//-}" >> "$GITHUB_OUTPUT"
+
             - name: Set up Docker Buildx
               uses: docker/setup-buildx-action@v4
 
@@ -29,4 +42,4 @@ jobs:
                 context: .
                 file: contrib/container/maptool/Dockerfile
                 push: true
-                tags: ghcr.io/${{ github.repository }}/maptool:trunk
+                tags: ghcr.io/${{ github.repository }}/maptool:${{ steps.image.outputs.tag }}