Duplicate environment variables for root

Duplicates the ENV statement to include all variables for root. It was
originally planned to only duplicate DEVKITPRO and DEVKITARM, but the
modification of PATH is potentially useful as well and maybe DISPLAY
too, so I see no reason not to duplicate the whole statement.

Closes #25. [skip ci]

Dockerfile

@@ -49,6 +49,11 @@ RUN REPO=http://cdn-fastly.deb.debian.org && \
 
     true;
 
+ENV DISPLAY=":0" \
+    DEVKITPRO="/opt/devkitPro" \
+    DEVKITARM="/opt/devkitPro/devkitARM" \
+    PATH="/opt/devkitPro/devkitARM/bin:${PATH}"
+
 USER user
 
 COPY ["imagefs/", "/"]