feat: execute service as distributed erlang (#96)

To create a cluster with distributed nodes, the following changes were made:

* Use [`dns_cluster`][0] to execute distributed nodes
* Add _script_ to execute distributed nodes locally

Also, improve local execution by:

* Use [`mix release`][1] to make easier [deploy `phoenix`][2]
* Use [`compose watch`][3] to synchronize code from the host with the service

Some minor improvements:

* Ignore files and folders in git and docker
* Remove boilerplate comments

[0]: https://github.com/phoenixframework/dns_cluster
[1]: https://hexdocs.pm/mix/Mix.Tasks.Release.html
[2]: https://hexdocs.pm/phoenix/releases.html
[3]: https://docs.docker.com/compose/how-tos/file-watch/

Co-authored-by: Joao P Dubas <joao.dubas@gmail.com>
Reviewed-on: #96
Co-authored-by: Joao P Dubas <joao.dubas+gitea@gmail.com>
Co-committed-by: Joao P Dubas <joao.dubas+gitea@gmail.com>

priv/docker/service/docker-entrypoint.sh

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+elixir --name ${HOSTNAME}@$(hostname -i | cut -d ' ' -f1) --cookie ${1} -S mix phx.server

priv/docker/service/docker-remote.sh

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+iex --name ex_trainer_remote@127.0.0.1 --hidden --cookie local-cookie --remsh ${HOSTNAME}@$(hostname -i | cut -d ' ' -f1)