Add minimal Dockerfile (#72)

Adds a Dockerfile that builds sally using a phased Docker build.
The first phase builds sally,
and the second phase publishes a scratch image
with just sally.

If Uber publishes this to a container registry,
a user can use it like so:

    % vim sally.yaml # create a sally yaml
    % cat > Dockerfile
    FROM sally:latest
    COPY sally.yaml /sally.yaml
    % docker build .

Even without publishing, this provides an example
of how to build sally for deployment.

Testing:
I verified the instructions above locally
by tagging locally with:

    % docker build -t sally:latest

I'm also using a variation of this Dockerfile in production
right now for my own hosted instance of sally.

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