add default image docker fallback

crates/devcontainer-rs/src/consts.rs

@@ -22,3 +22,5 @@ pub(crate) const CONTEXT_CHUNKS: usize = 4;
 /// Lectures successives du code de sortie d'un exec, et attente entre elles.
 pub(crate) const EXIT_CODE_ATTEMPTS: usize = 10;
 pub(crate) const EXIT_CODE_DELAY: Duration = Duration::from_millis(20);
+
+pub(crate) const DEFAULT_IMAGE_NAME: &str = "debian:stable-slim";