GUACAMOLE-2119: Add setting to enable reading _FILE variables by default.

guacamole-docker/entrypoint.d/100-generate-guacamole-home.sh

@@ -113,3 +113,18 @@ enable-environment-properties: true
 EOF
 fi
 
+# Enable reading of environment variables that pull values for properties
+# from files, unless overridden
+if ! is_property_set "enable-file-environment-properties"; then
+    cat >> "$GUACAMOLE_HOME/guacamole.properties" <<'EOF'
+#
+# NOTE: The following was automatically added by the container entrypoint to
+# allow Guacamole configuration property values to be automatically read from
+# files specified by environment variables ending in _FILE. If this is not
+# desired, you can override this behavior by specifying the
+# "enable-file-environment-properties" variable yourself in your
+# own guacamole.properties file.
+#
+enable-file-environment-properties: true
+EOF
+fi