Remove docker for local tests

Nobody has claimed this yet.

We should remove docker for local tests. I thought it'd help but it is more of a pain. I pretty much exclusively use crystal on my local machine.

I think next steps are:

* Change script/setup and script/test to use `shards install` and `crystal spec` and not docker
* Remove Dockerfile and related files
* Document how to use `config/database.local.cr` to setup local config

Here is an example of mine:

```crystal
database_name = "avram_test"

TestDatabase.configure do |settings|
settings.credentials = Avram::Credentials.parse?(ENV["DATABASE_URL"]?) || Avram::Credentials.new(
database: database_name,
hostname: ENV["DB_HOST"]? || "localhost",
port: ENV["DB_PORT"]?.try(&.to_i) || 5432,
# Some common usernames are "postgres", "root", or your system username (run 'whoami')
username: ENV["DB_USERNAME"]? || "postgres",
# Some Postgres installations require no password. Use "" if that is the case.
password: ENV["DB_PASSWORD"]? || "postgres"
)
end
```

Or maybe this should be the default, and we should include the `dotenv` shard in development mode so people can set up their vars there. I'd be fine either way

`script/setup` would do something like:

```
shards install
lucky db.setup
```

Start with script/setup and script/test, then inspect the Dockerfile and related Docker files. Compare the proposed shards install, crystal spec, and lucky db.setup flow with config/database.local.cr, and document the local PostgreSQL configuration; done means local tests no longer require Docker and setup instructions are clear.

Written by the indexing model from the issue text.