collapse `RUN` declaration in Dockerfile.

(see https://stackoverflow.com/questions/39223249/multiple-run-vs-single-chained-run-in-dockerfile-which-is-better)

Dockerfile

@@ -19,11 +19,11 @@ RUN npm i -g npm@>=7
 RUN npm i
 
 # Ensure these directories & files exist for compose volumes
-RUN mkdir -p /opt/ass/uploads/thumbnails/
+RUN mkdir -p /opt/ass/uploads/thumbnails/ && \
-RUN mkdir -p /opt/ass/share/
+    mkdir -p /opt/ass/share/ && \
-RUN touch /opt/ass/config.json
+    touch /opt/ass/config.json && \
-RUN touch /opt/ass/auth.json
+    touch /opt/ass/auth.json && \
-RUN touch /opt/ass/data.json
+    touch /opt/ass/data.json
 
 # Start ass
 CMD npm start