Fix: remove {} from RUN as this is not supported

The docker RUN uses /bin/sh as the default, this does not support curly braces. aura#308

Dockerfile

@@ -40,7 +40,11 @@ RUN adduser --home /app --no-create-home --system --uid ${AURA_UID} --group app
     # containers. We therefore create them here and set the permissions right away
     # since named volumes otherwise create these directories themself as the root
     # user.
-    mkdir -p /app/{logs,static/admin,site_media} && \
+    mkdir -p \
+    /app/site_media \
+    /app/static/admin \
+    /app/logs \
+    && \
     chown -R app:app /app
 
 USER app