Refector: Do the npm compile as a Build step

This removes the requirement of npm on the host system and removes an
additional manual step during the building of the docker image.

Updated the documentation accordingly and fixed a typo.

Dockerfile

@@ -1,3 +1,12 @@
+FROM node AS compile-frontend
+
+COPY . /src
+
+WORKDIR /src/src-ui
+RUN npm install
+RUN ./node_modules/.bin/ng build --prod
+
+
 FROM ubuntu:20.04 AS jbig2enc
 
 WORKDIR /usr/src/jbig2enc
@@ -88,7 +97,7 @@ RUN cd docker \
 COPY gunicorn.conf.py ../
 
 # copy app
-COPY src/ ./
+COPY --from=compile-frontend /src/src/ ./
 
 # add users, setup scripts
 RUN addgroup --gid 1000 paperless \