Update .env

[fcooper98]

No description provided.

[danintel]

This seems to go backward from bionic (Ubuntu 18) to xenial (Ubuntu 16).
Is this to solve a current problem with no stable Bionic release (yet)?

A workaround:
Change shell/Dockerfile line 19
From
RUN echo "deb http://repo.sawtooth.me/ubuntu/1.0/stable xenial universe" >> /etc/apt/sources.list \
To
RUN echo "deb http://repo.sawtooth.me/ubuntu/nightly bionic universe" >> /etc/apt/sources.list \

[danintel]

See comments. The change goes backwards (Bionic to Xenial, or in other words Ubuntu 18 to 16), not forwards. It is properly called a downgrade.