Switch to "node:17" as base image and install Python dependencies manually.

This saves about 220 MB in image size.

server/docker/Dockerfile

@@ -1,9 +1,14 @@
-FROM nikolaik/python-nodejs:python3.9-nodejs15
+FROM node:17
 
 ADD . /app
 ADD docker/config.js.docker /app/config.js
 
-RUN apt-get update || : && apt-get install pdftk python-pypdf2 -y
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        pdftk \
+        python3 \
+        python3-pip \
+        python3-pypdf2
 
 WORKDIR /app