Fix Dockerfile API_URL ARG not working in prod

The reason for this was that ARG values do not persist between build stages.
More info:
moby/moby#37345 (comment)

Dockerfile

@@ -17,19 +17,17 @@ CMD ["yarn", "dev"]
 
 FROM dev AS circleci
 
-# Needs to be during `yarn build`. Will be an empty string in circleci
-# and gets an actual value when building prod image.
-ARG API_URL
-
 COPY --chown=user:user . .
 
 CMD yarn lint \
     && yarn type-check \
-    && yarn build
+    && API_URL= yarn build
 
 
 FROM circleci as prod
 
+ARG API_URL
+
 RUN yarn build
 
 CMD ["yarn", "start"]