fix gpg error at build time

Dockerfile

@@ -88,8 +88,18 @@ RUN GPG_KEYS=B0F4253373F8F6F510D42178520A9993A1C052F8 \
   && curl -fSL https://github.com/simpl/ngx_devel_kit/archive/v$DEVEL_KIT_MODULE_VERSION.tar.gz -o ndk.tar.gz \
   && curl -fSL https://github.com/openresty/lua-nginx-module/archive/v$LUA_MODULE_VERSION.tar.gz -o lua.tar.gz \
   && export GNUPGHOME="$(mktemp -d)" \
-  && gpg --keyserver ha.pool.sks-keyservers.net --recv-keys "$GPG_KEYS" \
-  && gpg --batch --verify nginx.tar.gz.asc nginx.tar.gz \
+  && found=''; \
+  for server in \
+    ha.pool.sks-keyservers.net \
+    hkp://keyserver.ubuntu.com:80 \
+    hkp://p80.pool.sks-keyservers.net:80 \
+    pgp.mit.edu \
+  ; do \
+    echo "Fetching GPG key $GPG_KEYS from $server"; \
+    gpg --keyserver "$server" --keyserver-options timeout=10 --recv-keys "$GPG_KEYS" && found=yes && break; \
+  done; \
+  test -z "$found" && echo >&2 "error: failed to fetch GPG key $GPG_KEYS" && exit 1; \
+  gpg --batch --verify nginx.tar.gz.asc nginx.tar.gz \
   && rm -r "$GNUPGHOME" nginx.tar.gz.asc \
   && mkdir -p /usr/src \
   && tar -zxC /usr/src -f nginx.tar.gz \