Use "." as default-value for the --workspace-folder argument

[exhuma]

Considering @devcontainers/cli: Currently many commands require me to explicitly set --workspace-folder. This is redundant and cumbersome. I would wager that the "current folder" is a sensible default for most. Thus it would greatly improve usability if the argument would be optional with the default set to .

[samruddhikhandale]

Hi 👋

We have a similar request added to devcontainers/cli#29. Do you mind bumping it up with another comment? Thanks!