Update default container user

Dockerfile

@@ -14,11 +14,11 @@ EXPOSE 7860
 RUN mkdir -p /data
 RUN chmod 777 /data
 
-# The Space container runs with user ID 1000
-RUN useradd -m -u 1000 user
-ENV HOME="/home/user"
+# The Space container runs with user ID 1000, which corresponds to
+# ubuntu in the Livebook image.
+ENV HOME="/home/ubuntu"
 
-USER user
+USER ubuntu
 
-COPY --chown=user public-apps/ /public-apps
+COPY --chown=ubuntu public-apps/ /public-apps
 RUN /app/bin/warmup_apps