update dockerfile to use the latest release available (#14232)

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

build/Dockerfile

@@ -4,7 +4,7 @@ FROM alpine:3.11
 ENV PATH="/mattermost/bin:${PATH}"
 ARG PUID=2000
 ARG PGID=2000
-ARG MM_PACKAGE="https://releases.mattermost.com/5.18.1/mattermost-5.18.1-linux-amd64.tar.gz"
+ARG MM_PACKAGE="https://releases.mattermost.com/5.21.0/mattermost-5.21.0-linux-amd64.tar.gz"
 
 
 # Install some needed packages