docker: add runtime config injection from env

Dockerfile

@@ -5,6 +5,8 @@ FROM nginx:mainline
 
 # The safest way to build this image is to use `yarn docker-build`
 
+RUN apt-get update && apt-get -y install jq && rm -rf /var/lib/apt/lists/*
+
 COPY packages/app/dist /usr/share/nginx/html
 COPY docker/default.conf.template /etc/nginx/conf.d/default.conf.template
 COPY docker/run.sh /usr/local/bin/run.sh

docker/run.sh

@@ -1,9 +1,41 @@
 #!/usr/bin/env bash
 
+set -Eeuo pipefail
+
 # Run nginx as root
 sed -i 's/user  nginx.*$//' /etc/nginx/nginx.conf
 
 # Write selected env vars to nginx config
 envsubst '$PORT' < /etc/nginx/conf.d/default.conf.template > /etc/nginx/conf.d/default.conf
 
+# Inject runtime config into the client
+function inject_config() {
+  # Read runtime config from env in the same way as the @backstage/config-loader package
+  local config
+  config="$(jq -n 'env |
+    with_entries(select(.key | startswith("APP_CONFIG_")) | .key |= sub("APP_CONFIG_"; "")) |
+    to_entries |
+    reduce .[] as $item (
+      {}; setpath($item.key | split("_"); $item.value | fromjson)
+    )')"
+
+  >&2 echo "Runtime app config: $config"
+
+  local main_js
+  main_js="$(ls /usr/share/nginx/html/main.*.chunk.js)"
+  echo "Writing runtime config to ${main_js}"
+
+  # escape ' and " twice, for both sed and json
+  local config_escaped_1
+  config_escaped_1="$(echo "$config" | jq -cM . | sed -e 's/[\\"\x27]/\\&/g')" # \x27 = '
+  # escape / and & for sed
+  local config_escaped_2
+  config_escaped_2="$(echo "$config_escaped_1" | sed -e 's/[\/&]/\\&/g')"
+
+  # Replace __APP_INJECTED_RUNTIME_CONFIG__ in the main chunk with the runtime config
+  sed -e "s/__APP_INJECTED_RUNTIME_CONFIG__/$config_escaped_2/" -i "$main_js"
+}
+
+inject_config
+
 exec nginx -g 'daemon off;'