How does Docker construct image layers, and how does the OverlayFS union file system work under the hood?

Docker images are constructed as a stack of read-only layers. Each instruction in a Dockerfile (e.g., RUN, COPY, ADD) creates a new layer.

• Union File System: Merges all read-only layers into a single, unified filesystem view.
• Copy-On-Write (CoW): When a running container modifies a file in a read-only layer, Docker copies the file to the writable container layer before applying the edits, leaving the underlying image layers unchanged.
• OverlayFS: The union filesystem driver used by Docker. It combines:
  • Lowerdir: The read-only image layers.
  • Upperdir: The writable container layer containing modifications.
  • Merged: The combined view presented to the running container process.