feat: add retry logic and explicit Alpine repositories to gateway Dockerfile

Add explicit Alpine 3.21 main and community repository URLs to /etc/apk/repositories. Implement 5-attempt retry loop with 3-second delays for apk update and package installation to handle transient network failures during Docker builds.

deploy/gateway/Dockerfile

@@ -1,6 +1,16 @@
 FROM alpine:3.21
 
-RUN apk add --no-cache bash curl jq wireguard-tools nftables
+RUN set -eux; \
+    printf '%s\n' \
+      'https://dl-cdn.alpinelinux.org/alpine/v3.21/main' \
+      'https://dl-cdn.alpinelinux.org/alpine/v3.21/community' \
+      > /etc/apk/repositories; \
+    for attempt in 1 2 3 4 5; do \
+      apk update && apk add --no-cache bash curl jq wireguard-tools nftables && exit 0; \
+      echo "apk install attempt ${attempt} failed, retrying..." >&2; \
+      sleep 3; \
+    done; \
+    exit 1
 
 WORKDIR /app
 COPY scripts/gateway-entrypoint.sh /scripts/gateway-entrypoint.sh