backend/Dockerfile: retry apt-get on transient mirror 404s (#757)

Author: fangge518

backend/Dockerfile

@@ -6,8 +6,16 @@ ENV PYTHONUNBUFFERED=True
 ENV APP_HOME=/app
 WORKDIR $APP_HOME
 
-RUN apt-get update && \
-    apt-get install -y openssh-client libpq-dev postgresql libpq-dev gdal-bin libgdal-dev && \
+# Retries apt-get update+install a few times: deb.debian.org is a CDN in
+# front of many mirror origins, and a -security point-release rotation can
+# leave one edge's index listing a filename another edge has already
+# purged, producing a 404 that a fresh apt-get update usually resolves on
+# the next attempt (seen live: PR #723's preview build, 2026-09-04).
+RUN for i in 1 2 3 4 5; do \
+      apt-get update && \
+      apt-get install -y openssh-client libpq-dev postgresql libpq-dev gdal-bin libgdal-dev \
+      && break || { echo "apt-get attempt $i failed, retrying..."; sleep 10; }; \
+    done && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*