Support multi-select in 'containers' list

[eine]

When multiple stopped containers need to be removed, it would be useful to be able to select all of them with Ctrl or Shift and/or through checkboxes, and then right-click once only.

My use case is removal, but I believe the same feature can be useful for stopping, restarting, viewing the logs, attaching, etc.

[ejizba]

@1138-4eb this is already possible if you run "Docker Containers: Remove" from the command palette (View -> Command Palette)

It's not yet supported directly in the tree, but that's tracked here: #331