fix(ci): add retry loop to docker push (handles Gitea registry timeout)

The Gitea Container Registry responded with 'net/http: timeout
awaiting response headers' midway through the push. The image
uploads ~50MB of layers, and the registry can be slow under load.

Add a push_with_retry() shell function that retries each push up
to 3 times with 15s backoff. Most importantly, it retries for
both the 'latest' and the SHA tag, so a partial failure doesn't
leave the registry in a half-pushed state.

Uses POSIX sh-compatible while loop (no bashisms like {1..3}).

.gitea/workflows/ci.yml

@@ -83,9 +83,26 @@ jobs:
           echo "=== Tag ==="
           docker tag shot-crafter-calculator:ci gitea.danielarroyo.cl/proyectos/shot-crafter-calculator:latest
           docker tag shot-crafter-calculator:ci gitea.danielarroyo.cl/proyectos/shot-crafter-calculator:${SHORT_SHA}
-          echo "=== Push ==="
-          docker push gitea.danielarroyo.cl/proyectos/shot-crafter-calculator:latest
-          docker push gitea.danielarroyo.cl/proyectos/shot-crafter-calculator:${SHORT_SHA}
+
+          push_with_retry() {
+            local ref="$1"
+            local attempt=1
+            local max_attempts=3
+            while [ $attempt -le $max_attempts ]; do
+              echo "=== Push (attempt $attempt/$max_attempts): $ref ==="
+              if docker push "$ref"; then
+                return 0
+              fi
+              echo "::warning::Push of $ref failed, retrying in 15s..."
+              sleep 15
+              attempt=$((attempt + 1))
+            done
+            echo "::error::Push of $ref failed after $max_attempts attempts"
+            return 1
+          }
+
+          push_with_retry "gitea.danielarroyo.cl/proyectos/shot-crafter-calculator:latest"
+          push_with_retry "gitea.danielarroyo.cl/proyectos/shot-crafter-calculator:${SHORT_SHA}"
           echo "=== Logout ==="
           docker logout gitea.danielarroyo.cl
           echo "=== Done ==="