add default image docker fallback

crates/devcontainer-rs/src/errors.rs

@@ -47,6 +47,9 @@ pub enum ParseError {
     #[error("container file `{0}` does not exist or is not a regular file")]
     ContainerFileNotFound(PathBuf),
 
+    #[error("devcontainer file `{0}` declares neither `image` nor `build`")]
+    MissingImageOrBuild(PathBuf),
+
     #[error("the devcontainer file path has no parent directory: `{0}`")]
     InvalidDevContainerPath(PathBuf),
 }