Update base container image from alpine 3.12 to 3.14 (#18450)

* update default mm package to 5.39.0

* update base container image from alpine 3.12 to 3.14

build/Dockerfile

@@ -1,10 +1,10 @@
-FROM alpine:3.12
+FROM alpine:3.14
 
 # Some ENV variables
 ENV PATH="/mattermost/bin:${PATH}"
 ARG PUID=2000
 ARG PGID=2000
-ARG MM_PACKAGE="https://releases.mattermost.com/5.38.1/mattermost-5.38.1-linux-amd64.tar.gz?src=docker"
+ARG MM_PACKAGE="https://releases.mattermost.com/5.39.0/mattermost-5.39.0-linux-amd64.tar.gz?src=docker"
 
 
 # Install some needed packages