fix(docker): harden frontend runtime config

docker/frontend-entrypoint.sh

@@ -2,8 +2,13 @@
 set -eu
 
 backend_base_url=${BACKEND_BASE_URL:-/api}
-escaped_backend_base_url=$(printf '%s' "$backend_base_url" | sed 's/[&|\\]/\\&/g')
-sed "s|__BACKEND_BASE_URL__|${escaped_backend_base_url}|g" \
-  /usr/share/nginx/html/config.template.js > /usr/share/nginx/html/config.js
+runtime_config=$(jq -cn --arg backend_base_url "$backend_base_url" \
+  '{backendBaseUrl: $backend_base_url}')
+printf 'window.__THOTHII_CONFIG__ = %s;\n' "$runtime_config" \
+  > /usr/share/nginx/html/config.js
+
+if [ "$#" -gt 0 ]; then
+  exec "$@"
+fi
 
 exec nginx -g 'daemon off;'