[feat] add options from mumble-server.ini

Most of the options from mumble-server.ini have been added to .env
No documentation for these settings, since they're described elsewhere.

The exceptions which were not included are:
- *ssl* : adding support for custom certs needs more work
- database, *db*, *sqlite* : as above, for databases
- *ice* : unclear how ZeroC Ice works, decided to skip for now
- logfile, pidfile, *log* : would need to expose these files outside the
  container
- port, host, uname : unneccessary in a container
- legacypasswordhash : unsecure, therefore requires custom compose file
- forceExternalAuth : undocumented

Reference:
on environment variables to set config options in the container: a0ab293b2a/README.md (configuration)
list of options: https://github.com/mumble-voip/mumble/blob/v1.6.870/auxiliary_files/mumble-server.ini

.env.sample

@@ -21,3 +21,59 @@ COMPOSE_FILE="compose.yml"
 
 ## SuperUser Password
 SECRET_SUPERUSER_PASSWORD_VERSION=v1
+
+#################
+# MUMBLE CONFIG #
+#################
+# Documentation for the following options is here:
+# https://github.com/mumble-voip/mumble/blob/v1.6.870/auxiliary_files/mumble-server.ini
+# Note: these options may be untested
+#WELCOME_TEXT=
+#BANDWIDTH=
+#TIMEOUT=
+#USERS=
+#USERS_PER_CHANNEL=
+#MESSAGE_BURST=
+#MESSAGE_LIMIT=
+#PLUGIN_MESSAGE_LIMIT=
+#PLUGIN_MESSAGE_BURST=
+#ALLOW_PING=
+#OPUS_THRESHOLD=
+#CHANNEL_NESTING_LIMIT=
+#CHANNEL_COUNT_LIMIT=
+#CHANNEL_NAME=
+#USERNAME=
+#DEFAULT_CHANNEL=
+#REMEMBER_CHANNEL=
+#REMEMBER_CHANNEL_DURATION=
+#TEXT_MESSAGE_LENGTH=
+#IMAGE_MESSAGE_LENGTH=
+#ALLOW_HTML=
+#LOG_DAYS=
+#REGISTER_NAME=
+#REGISTER_PASSWORD=
+#REGISTER_URL=
+#REGISTER_HOSTNAME=
+#REGISTER_LOCATION=
+#BONJOUR=
+#SSL_CERT=
+#SSL_KEY=
+#SSL_PASSPHRASE=
+#SSL_CA_PEM=
+#SSL_DH_PARAMS=
+#SSL_CIPHERS=
+#OBFUSCATE=
+#CERT_REQUIRED=
+#SEND_VERSION=
+#SUGGEST_VERSION=
+#SUGGEST_POSITIONAL=
+#SUGGEST_PUSH_TO_TALK=
+#KDF_ITERATIONS=
+#AUTOBAN_ATTEMPTS=
+#AUTOBAN_TIMEFRAME=
+#AUTOBAN_TIME=
+#AUTOBAN_SUCCESSFUL_CONNECTIONS=
+#ALLOW_RECORDING=
+#ROLLING_STATS_WINDOW=
+#LISTENERS_PER_CHANNEL=
+#LISTENERS_PER_USER=