vscode sphinx-build task in virtual environment

I am using virtualenv in vscode and I'd like to have a task to build the documentation using sphinx-build. My tasks.json contains

"tasks": [
    {
        "label": "Build Docs",
        "type": "shell",
        "command": "sphinx-build",
        "args": ["./sphinx", "./docs/sphinx_build"],
        "options": {
            "cwd": "${workspaceFolder}"
        }
    }

However, when I run this it seems that vscode is not using the Scripts folder of my virtual environment, so I get

sphinx-build : The term 'sphinx-build' is not recognized as the name of a cmdlet, function, script file, or operable program.

The same command runs just fine in the terminal. Am I missing something here? Is there a way to correctly point the task to the correct location in a virtual environment?