Using INVENTREE_PY_ENV does not install requirements in venv with docker

Please verify that this bug has NOT been raised before.

• I checked and didn't find a similar issue

Describe the bug*

You can't use a custom venv with a docker installation as the requirements are not installed automatically by the init script.

Steps to Reproduce

1. Set the INVENTREE_PY_ENV env-var for the Docker container
2. Start container
3. Inspect the venv content

Expected behaviour

The requirements should have been installed

Deployment Method

Docker

Version Information

1.5.2

Try to reproduce on the demo site

I tried to reproduce

Is the bug reproducible on the demo site?

Not reproducible

Relevant log output