feat: `docker run` option

[ftupas]

This pr does the following:

• Adds Dockerfile to build the rainfrog binary
• Update readme
• resolves #89

❯ docker run -it --rm --name rainfrog \
  -p 5432:5432 \
  -e username="rainfrog" \
  -e password="password" \
  -e hostname="host.docker.internal" \
  -e db_port="5432" \
  -e dbname="rainfrog" rainfrog