Use OverlayFS to persist changes to workspace HOME directory

MR: Pending

Description

Use OverlayFS to persist changes to workspace HOME directory.

1. Leverage the OverlayFS filesystem
2. Use OverlayFS to mount the contents of the container's existing HOME directory as a "lower layer" (read only), and a directory on the existing persistent volume as the "upper layer" (writeable).
3. Inject commands/script to be run at container start executes the proper mkdir ... and mount -t overlay ... commands to achieve the above desired OverlayFS mount setup.
4. Execution of these commands/script at container start will achieved be via the support for running specific commands in containers which we are adding as part of Startup scripts for Remote Development workspac... (&15602)

Acceptance Criteria

• HOME directory of all containers in a workspaces are persisted across workspace stop/start.
• Relevant documentation updated
• Test with many and/or large files in the HOME directory, and ensure that the overlayfs does not negatively impact performance.

Technical Requirements

• See description for overlayfs details
• Ensure that we use the existing persistent volume.

Design Requirements

No UI changes, this should be transparent to the user.

Impact Assessment

Users no longer lose changes in HOME directory when workspace is restarted.