backend/Dockerfile: retry apt-get on transient mirror 404s

deb.debian.org is a CDN in front of many mirror origins; a -security
point-release rotation can leave one edge's index listing a filename
another edge has already purged, producing a 404 an apt-get install
can't recover from mid-run. Wrapping update+install in a retry loop
lets a fresh apt-get update (which usually lands on a consistent edge)
resolve it within the same build, instead of needing a whole CI job
rerun.

Verified live: this branch's AWS Preview workflow hit this exact
failure 3 times in a row on 3 different packages (libtiff, libc-l10n,
libperl); reproduced locally against the unmodified Dockerfile
(libde265-0, arm64) while testing the fix. Confirmed the retry loop
itself works (recovered from the same live mirror flakiness during a
local arm64 build) and that the full build succeeds end-to-end on
linux/amd64 (the real CI architecture) with the fix applied.

No deploy-api.yml failure has ever been caused by this — checked its
last ~30 runs; its only 2 failures were unrelated migration-task
issues, apt-get succeeded in both. Folding this into #723 rather than
a separate PR so the fix gets verified through this branch's own
preview deploy, which is already hitting the failure live.

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/*