Use npm ci instead of npm install

Refs #394

Because we have the `package-lock.json` checked in, `npm ci` is
favorable.  If one desires to add or update a package, the intended
workflow is to change the `package.json`, execute `npm install
locally, and check in the appropriate changes in the lock file.

docker/dev/container/hooks/init/20_npm

@@ -8,4 +8,4 @@ if [[ -d node_modules ]] && $(shopt -s nullglob dotglob; files=(node_modules/*);
 	exit
 fi
 
-npm install
+npm ci

docker/prod.Dockerfile

@@ -12,7 +12,7 @@ RUN /opt/pycroft/venv/bin/pip wheel --wheel-dir /opt/pycroft/wheel -r requiremen
 
 # Download JS/CSS dependencies
 COPY --chown=pycroft:pycroft package.json .
-RUN npm install
+RUN npm ci
 
 # Build Pycroft wheel
 COPY --chown=pycroft:pycroft . .