Dockerfile: provide a secret to allow build to proceed

Secret is not used in build time. However, config.js
requires one. It won't affect runtime secret in any way
and "flood" used here will be blocked in runtime.

Dockerfile

@@ -26,7 +26,7 @@ COPY server ./server
 COPY shared ./shared
 COPY scripts ./scripts
 COPY config.docker.js ./config.js
-RUN npm run build && \
+RUN FLOOD_SECRET=flood npm run build && \
     npm prune --production
 
 # Now get the clean image without any dependencies and copy compiled app