bump dockerfile with latest mattermost package (#11901)

build/Dockerfile

@@ -4,7 +4,7 @@ FROM alpine:3.10
 ENV PATH="/mattermost/bin:${PATH}"
 ARG PUID=2000
 ARG PGID=2000
-ARG MM_PACKAGE="https://releases.mattermost.com/5.12.4/mattermost-5.12.4-linux-amd64.tar.gz"
+ARG MM_PACKAGE="https://releases.mattermost.com/5.14.0/mattermost-5.14.0-linux-amd64.tar.gz"
 
 
 # Install some needed packages