feat: add mix task for running tests under testcontainers

[jarlah]

run task in app with

mix testcontainers.test

or

mix testcontainers.test --database mysql

env vars will be populated with:

• DB_HOST
• DB_USER
• DB_PASSWORD
• DB_PORT

these env variables can be referenced in any of the config files, like dev.exs and test.exs like:

config :my_project, MyProject.Repo,
  username: System.get_env("DB_USER") || "postgres",
  password: System.get_env("DB_PASSWORD") || "postgres",
  hostname: System.get_env("DB_HOST") || "localhost",
  port: System.get_env("DB_PORT") || "5432",
  database: "my_project_dev",
  stacktrace: true,
  show_sensitive_data_on_connection_error: true,
  pool_size: 10

this will completely remove the need for the phoenix application.ex hack.

•  Remove/deprecate phoenix hack (ecto.ex) and update README