DOPS-215: Add MM_INSTALL_TYPE environment variable. (#497)

app/Dockerfile

@@ -3,6 +3,7 @@ FROM alpine:3.10
 # Some ENV variables
 ENV PATH="/mattermost/bin:${PATH}"
 ENV MM_VERSION=5.27.0
+ENV MM_INSTALL_TYPE=docker
 
 # Build argument to set Mattermost edition
 ARG edition=enterprise