Dockerfile: retry apk add on transient Alpine mirror TLS failures

Dockerfile

@@ -5,7 +5,14 @@ COPY package.json package-lock.json* ./
 RUN npm install --omit=dev
 COPY . .
 RUN mkdir -p uploads certs
-RUN apk add --no-cache openssl && \
+RUN set -e; \
+    i=1; \
+    until apk add --no-cache openssl; do \
+      echo "apk add failed on attempt $i, retrying in 5s..."; \
+      i=$((i+1)); \
+      if [ $i -gt 6 ]; then echo "apk add failed after 6 attempts"; exit 1; fi; \
+      sleep 5; \
+    done && \
     openssl req -x509 -nodes -newkey rsa:2048 -days 3650 \
     -keyout certs/key.pem -out certs/cert.pem \
     -subj "/CN=whatido.local" -addext "subjectAltName=DNS:localhost,IP:127.0.0.1"