ci: image job on the dedicated 'echolot' runner; public registry host default

compilesau-echolot (repo runner, label echolot) handles the docker image
build. Registry host defaults to git.rambossek.at (runner-internal
GITHUB_SERVER_URL is http://app:3000). Asset re-uploads on tag re-runs
warn instead of failing the completed release.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/build-server.yml

@@ -55,12 +55,15 @@ jobs:
           # the uploads 404 — that broke the first v0.1.0 release run.
           ID=$(echo "$REL" | grep -o '"id":[0-9]*' | head -1 | cut -d: -f2)
           for f in dist/*; do
+            # Tolerate re-runs: an existing asset of the same name may 4xx.
             curl -sf -X POST "$API/releases/$ID/assets?name=$(basename "$f")" \
-              -H "Authorization: token $TOKEN" -F "attachment=@$f"
+              -H "Authorization: token $TOKEN" -F "attachment=@$f" \
+              || echo "::warning::upload of $(basename "$f") failed (already attached?)"
           done
 
   image:
-    runs-on: ubuntu-latest
+    # Dedicated docker-capable repo runner ("compilesau-echolot").
+    runs-on: echolot
     steps:
       - uses: actions/checkout@v4
 
@@ -69,9 +72,9 @@ jobs:
         run: |
           echo "version=${GITHUB_REF_NAME#server-}" >> "$GITHUB_OUTPUT"
           # GITHUB_SERVER_URL inside the runner is the INTERNAL url
-          # (http://app:3000) — strip any scheme, and prefer the public host
-          # for the registry so pulled image references work outside.
-          HOST="${ECHOLOT_REGISTRY_HOST:-${GITHUB_SERVER_URL#*://}}"
+          # (http://app:3000); the registry needs the public host so pulled
+          # image references work outside. Env var overrides if it changes.
+          HOST="${ECHOLOT_REGISTRY_HOST:-git.rambossek.at}"
           echo "host=$HOST" >> "$GITHUB_OUTPUT"
           echo "image=$HOST/${GITHUB_REPOSITORY,,}-server" >> "$GITHUB_OUTPUT"