fix: install npm dependencies for library assets before building

The library's JavaScript code depends on @floating-ui/dom package
which is listed in assets/package.json. Need to run npm install
before building the library assets with esbuild.

Author: ukutaht

demo/Dockerfile

@@ -47,6 +47,7 @@ COPY config /prima/config
 # Build prima library assets
 WORKDIR /prima
 RUN mix deps.get --only prod
+RUN npm install --prefix ./assets
 RUN mix assets.setup
 RUN mix assets.build