Fix installing default venv packages (#18)

* Fix installing default venv packages
* Use Bash as shell for Dockerfile

Dockerfile

@@ -1,5 +1,8 @@
 FROM ubuntu:20.04
 
+# Use Bash shell for Python venv script
+SHELL ["/bin/bash", "-c"]
+
 # To make it easier for build and release pipelines to run apt-get,
 # configure apt to not require confirmation (assume the -y argument by default)
 ENV DEBIAN_FRONTEND=noninteractive
@@ -33,6 +36,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 && python3 -m venv /ado-venv \
 # Make this the default venv for root
 && echo "source /ado-venv/bin/activate" >> ~root/.bashrc \
+&& source /ado-venv/bin/activate \
 && pip install wheel \
 && pip install ansible==5.2.0 foodx-devops-tools==0.12.1 \
 # Install .NETCore runtime dependency for the agent