feat: support config via systemd credentials (#20406)

feat: Support config via Systemd Credentials

See https://systemd.io/CREDENTIALS/. This is used as a fallback, so will
only be used if the `$*_FILE` var is empty. This could also be used to
implicitly use Docker Secrets by settings
`CREDENTIALS_DIRECTORY=/run/secrets` rather than setting individual
`$_*FILE` environment variables.

server/bin/start.sh

@@ -11,8 +11,12 @@ export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/usr/lib/jellyfin-ffmpeg/lib"
 SERVER_HOME=/usr/src/app/server
 
 read_file_and_export() {
-	if [ -n "${!1}" ]; then
-		content="$(cat "${!1}")"
+	fname="${!1}"
+	if [[ -z $fname ]] && [[ -e "$CREDENTIALS_DIRECTORY/$2" ]]; then
+		fname="${CREDENTIALS_DIRECTORY}/$2"
+	fi
+	if [[ -n $fname ]]; then
+		content="$(< "$fname")"
 		export "$2"="${content}"
 		unset "$1"
 	fi