Can you backup photos stored on external hard drive to the icloud

You are asking about a backup of the photos on an external volume.

A backup would mean an extra safety copy of your Photos library, so you can recover accidentally deleted photos or albums. Then the answer would be "No". You must not store a Photos Library in any folder, that is syncing with a cloud service (iCloud Drive, Google Drive, DropBox). It will damage the library.

What you can do, is setting up your Photos Library on the external volume as your iCloud Photos Library and have the library sync with iCloud, so you can access the library from other Apple Devices. But it will not be a backup. You will still have to include the Photos Library in your regular Time Machine backups or your other backup scheme.

See: Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community

If they are part of your local Photos library, yes.

Will it still work if the hard drive is plugged into a dock?

No way to know this until you try. "Dock" could mean anything. If your Mac sees it, you should be fine.