add heroku deployment steps

Dockerfile

@@ -15,3 +15,10 @@ RUN yarn build
 FROM nginx:mainline
 
 COPY --from=builder /app/packages/app/build /usr/share/nginx/html
+
+# Run nginx as root
+RUN sed -i 's/user  nginx.*$//' /etc/nginx/nginx.conf
+
+# Copy in the nginx conf template and replace "$PORT" with the environment variable set by heroku
+COPY default.conf.template /etc/nginx/conf.d/default.conf.template
+CMD /bin/bash -c "envsubst '\$PORT' < /etc/nginx/conf.d/default.conf.template > /etc/nginx/conf.d/default.conf" && nginx -g 'daemon off;'

default.conf.template

@@ -0,0 +1,8 @@
+server {
+  listen $PORT default_server;
+
+  location / {
+    root   /usr/share/nginx/html;
+    index  index.html;
+  }
+}