image: bump default MM package to use release 5.26.0 (#15276)

build/Dockerfile

@@ -4,7 +4,7 @@ FROM alpine:3.12
 ENV PATH="/mattermost/bin:${PATH}"
 ARG PUID=2000
 ARG PGID=2000
-ARG MM_PACKAGE="https://releases.mattermost.com/5.25.2/mattermost-5.25.2-linux-amd64.tar.gz?src=docker"
+ARG MM_PACKAGE="https://releases.mattermost.com/5.26.0/mattermost-5.26.0-linux-amd64.tar.gz?src=docker"
 
 
 # Install some needed packages