Dockerfile: fix entrypoint warnings

Entrypoint is a JSON array now; also it's entirely uppercase to address
docker build warnings.

Signed-off-by: Thilo Fromm <[email protected]>

Author: t-lo

Dockerfile

@@ -63,4 +63,4 @@ RUN addgroup -g 2001 mailuser \
 COPY scripts/ /
 RUN chmod 755 /*.sh
 
-entrypoint /entry.sh
+ENTRYPOINT [ "/entry.sh" ]