Dockerfile for checker

[vbrown608]

Depends on #206

I decided to give the checker its own Dockerfile because it builds a different binary and could have different dependencies. We could build two binaries in the existing Dockerfile but it seemed more consistent with the Docker philosophy to have a single-application image.

This can run daily scans with a checker/docker-compose.yml like

services:
    app:
        build: .
        environment:
          - CONNECTION_POOL_SIZE=32
        volumes:
          - ./shared:/shared
        command: /bin/sh -c 'env >> /etc/environment && crontab /shared/check.cron && crond -f'

and a checker/shared/check.cron like

0 15 * * * /go/bin/starttls-check --file=/shared/majestic_million.csv --column=2 --aggregate=true >> /shared/stats.csv

I'd like to commit those to a deploy repo managed by techops.

[sydneyli]

The changes from the last two commits look good to me.