Updated compose to build the docker image when required. Added env customisation options. Added update script

.env.example

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+# This defines the way to get the moonlight docker image
+# By default moonlight will build the image locally. To pull
+# it from the registry, set the value to 'pull'
+MOONLIGHT_BUILD=build
+
+# This defines the image name to use when building/pulling
+# the moonlight docker image
+MOONLIGHT_IMAGE=moonlightpanel/panel:2.1
+
+# Specify where the fetch the moonlight source code when building
+# Uncomment this if you want to select a specific version of moonlight
+# Or you want to deploy a custom fork
+# MOONLIGHT_REPOSITORY=https://github.com/Moonlight-Panel/Moonlight
+# MOONLIGHT_BRANCH=v2_ChangeArchitecture
+
+# Set this to the url moonlight is accessible through the internet
+MOONLIGHT_URL=http://localhost:9069
+
+# This defines the port moonlight should run on
+MOONLIGHT_PORT=9069
+
+# Here you can adjust where the moonlight instance will save
+# the application and database data
+MOONLIGHT_DATA=./data
+
+# With the following settings you can adjust the database configuration
+MOONLIGHT_DATABASE_USER=moonlight
+MOONLIGHT_DATABASE_PASSWORD=s3cret
+MOONLIGHT_DATABASE_NAME=moonlight