Dockerfile: fall back to edge.kernel.org mirror when dl-cdn TLS fetch fails

Dockerfile

@@ -6,9 +6,11 @@ RUN npm install --omit=dev
 COPY . .
 RUN mkdir -p uploads certs
 RUN set -e; \
+    V="v$(cut -d. -f1-2 /etc/alpine-release)"; \
     i=1; \
     until apk add --no-cache openssl; do \
-      echo "apk add failed on attempt $i, retrying in 5s..."; \
+      echo "apk add failed on attempt $i, switching to backup mirror..."; \
+      printf "https://mirrors.edge.kernel.org/alpine/%s/main\nhttps://mirrors.edge.kernel.org/alpine/%s/community\n" "$V" "$V" > /etc/apk/repositories; \
       i=$((i+1)); \
       if [ $i -gt 6 ]; then echo "apk add failed after 6 attempts"; exit 1; fi; \
       sleep 5; \