Install `npm` in the Docker image

`npm` is still used by the final deployment process to publish packages
to the npm registry.

Dockerfile

@@ -10,6 +10,7 @@ RUN apk update && apk add --no-cache \
   git \
   make \
   nodejs \
+  npm \
   yarn
 
 # Do not download a Chrome build as part of installing the "puppeteer" package,