ci: support manual backfilling of past releases in Docker workflow

Add a tag input to manual triggers to allow building and publishing specific past versions without clobbering the "latest" image tag.

.gitea/workflows/docker-publish.yml

@@ -7,6 +7,13 @@ on:
       # ...); only the app package's tag corresponds to this image.
       - "irtt-rs/v*"
   workflow_dispatch:
+    inputs:
+      tag:
+        description: >-
+          Existing irtt-rs/vX.Y.Z tag to build and publish (backfill a past
+          release). Leave empty to build the ref this run was dispatched from.
+        required: false
+        type: string
 
 env:
   IMAGE_NAME: irtt-server
@@ -22,19 +29,35 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.inputs.tag || github.ref }}
 
       - name: Determine image tags
         id: meta
         run: |
-          if [ "${GITHUB_REF_TYPE}" = "tag" ]; then
+          backfill_tag="${{ github.event.inputs.tag }}"
-            version="${GITHUB_REF_NAME#irtt-rs/v}"
+          if [ -n "${backfill_tag}" ]; then
+            ref_name="${backfill_tag}"
+            # Backfilling a past release must never move "latest" onto an
+            # older build just because it's being published after the fact.
+            push_latest=false
+          else
+            ref_name="${GITHUB_REF_NAME}"
+            push_latest=true
+          fi
+
+          if [[ "${ref_name}" == irtt-rs/v* ]]; then
+            version="${ref_name#irtt-rs/v}"
+          else
+            # Manual dispatch off a branch, no tag given: tag by commit only.
+            version="sha-$(git rev-parse --short HEAD)"
+            push_latest=false
+          fi
+
           echo "version=${version}" >> "$GITHUB_OUTPUT"
+          if [ "${push_latest}" = true ]; then
             echo "tags=${version},latest" >> "$GITHUB_OUTPUT"
           else
-            # Manual dispatch off a branch: tag by commit only, never touch
-            # "latest", so an ad-hoc build can't clobber the last release.
-            version="sha-$(git rev-parse --short HEAD)"
-            echo "version=${version}" >> "$GITHUB_OUTPUT"
             echo "tags=${version}" >> "$GITHUB_OUTPUT"
           fi