Test that we pull in env host variables

Now that clear env variables specified on the command line we can check
that values specified as `${VAR}` are pulled in from the host.

test/integration/docker/vm/Dockerfile

@@ -7,6 +7,8 @@ RUN apt-get update --fix-missing && apt-get -y install openssh-client openssh-se
 RUN mkdir /root/.ssh && ln -s /shared/ssh/id_rsa.pub /root/.ssh/authorized_keys
 RUN mkdir -p /etc/docker/certs.d/registry:4443 && ln -s /shared/certs/domain.crt /etc/docker/certs.d/registry:4443/ca.crt
 
+RUN echo "HOST_TOKEN=abcd" >> /etc/environment
+
 COPY boot.sh .
 
 HEALTHCHECK --interval=1s CMD pgrep dockerd