Edit Dockerfile (#763)

Co-authored-by: Jonas Chopin-Revel <contact@jonas-chopin.com>

Dockerfile

@@ -20,4 +20,6 @@ COPY --from=nim /src/nitter/nitter ./
 COPY --from=nim /src/nitter/nitter.example.conf ./nitter.conf
 COPY --from=nim /src/nitter/public ./public
 EXPOSE 8080
+RUN adduser -h /src/ -D -s /bin/sh nitter
+USER nitter
 CMD ./nitter