MAINTAINERS: Add Felix Singer to util/docker

Change-Id: Ic2efbb1d2a13212921ad110314a6394a4dca6a8a
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72918
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 7d2c621..64c93e5 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1001,6 +1001,7 @@
 
 DOCKER
 M:	Martin Roth <gaumless@gmail.com>
+M:	Felix Singer <felixsinger@posteo.net>
 S:	Supported
 F:	util/docker/