This shows the basics of the setup needed to participate in the Cygnified AoC with your solution for a given day (in this case day 19 because we are in the day19 directory). The directory structure is important.
The setup must include a Dockerfile that includes the solution source code. Your Dockerfile must include a COPY statement that copies input.txt (and possibly other files) from the root of day02 (in this case) into your image. That is, we will use your individual puzzle input provided by Advent of Code when analysing your solution.
From all this magic setup, the Cygnified AoC-engine can create a valid image and you're in the game (and that's awesome).
Shell scripting is fun and all, but we have provided examples for many different languages to get you up and running with your favourite weapons, have a look here
docker build -t aoc19 .
The environment variable part specifies which part of the solution to run.
docker run -e part=part1 aoc19
As this is just a template, all that part1 and part2 does is to print the first and last line of input.txt, respectively. For a more realistic example, have a look here.