Docker fix (#8835)

* Fix server command in Dockerfile

* Ensure invoke is installed into the venv

* Run extra check in docker build step

* Improve documentation

* Intercept ModuleNotFoundError

- Clear error message

* Docs updates

* Add extra check to dev docker build

* Cleanup tasks.py

* Prevent double activation of venv

* Change order of operations

---------

Co-authored-by: Matthias Mair <code@mjmair.com>

contrib/container/init.sh

@@ -40,14 +40,17 @@ if [[ -n "$INVENTREE_PY_ENV" ]]; then
     if test -d "$INVENTREE_PY_ENV"; then
         # venv already exists
         echo "Using Python virtual environment: ${INVENTREE_PY_ENV}"
+        source ${INVENTREE_PY_ENV}/bin/activate
     else
-        # Setup a virtual environment (within the "data/env" directory)
+        # Setup a virtual environment (within the provided directory)
         echo "Running first time setup for python environment"
         python3 -m venv ${INVENTREE_PY_ENV} --system-site-packages --upgrade-deps
+
+        # Ensure invoke tool is installed locally
+        source ${INVENTREE_PY_ENV}/bin/activate
+        python3 -m pip install --ignore-installed --upgrade invoke
     fi
 
-    # Now activate the venv
-    source ${INVENTREE_PY_ENV}/bin/activate
 fi
 
 cd ${INVENTREE_HOME}