Container couldn't be run to listen to the root directory (`/`)

[KharchenkoMaks]

Unfortunately, the container couldn't be run to listen to the root directory (/) at this time. Setting the WPR_VIRTUAL_DIR environment variable to an empty value reverts to the default value of wscservice. We need to implement the ability to listen to the root directory.

Test plan:

• Use root dir WPR_VIRTUAL_DIR=/
• Use empty dir WPR_VIRTUAL_DIR="" (same result)
• Verify default virtual dir is still wscservice
• Use custom virtual directory
• Use virtual dir with leading/trailing slashes, e.g. WPR_VIRTUAL_DIR=/wscservice/
• Verify WPR_VIRTUAL_DIR using both build argument and environment variable