fix user permission specific problems and add comments

Dockerfile

@@ -88,7 +88,7 @@ RUN chown -R build:build /home/build
 VOLUME /home/build/android
 VOLUME /srv/ccache
 
-CMD /home/build/startup.sh
-
 USER build
 WORKDIR /home/build/android
+
+CMD /home/build/startup.sh

run.sh

@@ -30,6 +30,9 @@ while [[ $# > 0 ]]; do
 done
 
 # Create shared folders
+# Although Docker would create non-existing directories on the fly,
+# we need to have them owned by the user (and not root), to be able
+# to write in them, which is a necessity for startup.sh
 mkdir -p $SOURCE
 mkdir -p $CCACHE
 

startup.sh

@@ -6,7 +6,9 @@ if [ ! -f ${CCACHE_DIR}/ccache.conf ]; then
 	ccache -M ${CCACHE_SIZE}
 fi
 
-export USER="build"
+# in Docker, the USER variable is unset by default
+# but some programs (like jack toolchain) rely on it
+export USER="$(whoami)"
 
 # Launch screen session
 screen -s /bin/bash