dockerfile: bump base MM package to 5.23.1 (#14757)

Automatic Merge

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/5.22.2/mattermost-5.22.2-linux-amd64.tar.gz?src=docker"
+ARG MM_PACKAGE="https://releases.mattermost.com/5.23.1/mattermost-5.23.1-linux-amd64.tar.gz?src=docker"
 
 
 # Install some needed packages