refactor: update m4b_tool_bin initialization and enhance argument parsing for backend selection

to-m4b.sh

@@ -4,12 +4,12 @@ set -eufo pipefail
 
 # CONSTANTS
 declare -r script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-declare -ra m4b_tool_bin=(nix run github:sandreas/m4b-tool#m4b-tool-libfdk --)
+declare -a m4b_tool_bin
 
 # VARS
 LEADING_ZEROES=2
-SRC="${script_dir}/src"
-OUT="${script_dir}/out"
+SRC=""
+OUT=""
 
 # FUNCTIONS
 parse-vars() {
@@ -226,4 +226,28 @@ main() {
   fi
 }
 
+# Argument parser: select backend (docker|nix) and set paths
+case "${1-}" in
+  nix)
+    m4b_tool_bin=(nix run github:sandreas/m4b-tool#m4b-tool-libfdk --)
+    SRC="${script_dir}/src"
+    OUT="${script_dir}/out"
+    shift
+    ;;
+  docker|"")
+    m4b_tool_bin=(docker run -it --rm -u $(id -u):$(id -g) -v "$(pwd)":/mnt sandreas/m4b-tool:latest)
+    SRC="./src"
+    OUT="./out"
+    [[ "${1-}" == docker ]] && shift || true
+    ;;
+  -h|--help)
+    echo "Usage: $0 [docker|nix]" >&2
+    exit 0
+    ;;
+  *)
+    echo "Unknown mode '$1'. Use 'docker' or 'nix'." >&2
+    exit 2
+    ;;
+esac
+
 main "$@"