contrib: Dockerfile: use UID/GID 100 for gluon user

To make the container behave the same regardless of the UIDs used
outside of the container, choose a UID that is unlikely to be used by
the user mounting their build directory the container. 100 should not
conflict with common defaults on Linux (min UID 1000) or MacOS X
(min UID 500). Conveniently, there is already a group "users" at GID
100, so creating a new group can be avoided.

contrib/docker/Dockerfile

@@ -30,7 +30,7 @@ RUN mkdir /tmp/ec &&\
     mv bin/ec-linux-amd64 /usr/local/bin/editorconfig-checker &&\
     rm -rf /tmp/ec
 
-RUN useradd -m -d /gluon gluon
+RUN useradd -m -d /gluon -u 100 -g 100 gluon
 USER gluon
 
 VOLUME /gluon