Turns mix release into a Lambda package builder.

# mix.exs
def project do
  [
    releases: [
      lambda: [
        steps: [&Mayfly.Release.prepare/1, :assemble, &Mayfly.Release.bootstrap/1, &Mayfly.Release.zip/1],
        mayfly: [handler: MyApp.Handler]
      ]
    ]
  ]
end

MIX_ENV=prod mix release lambda      # -> _build/prod/rel/lambda/lambda.zip

Use function captures, not a call such as Mayfly.Release.steps(): mix.exs is evaluated before dependencies are compiled, and a capture of a remote function does not require the module to be loaded, so mix deps.get on a fresh clone works. Drop &Mayfly.Release.zip/1 if you only want the directory.

:mayfly options

• :handler (required) – module implementing Mayfly.Handler or a "Module.function" string. Written into bootstrap as the default _HANDLER; the Lambda Handler setting overrides it.
• :layer – build for the Mayfly ERTS layer: sets include_erts: false and makes bootstrap use /opt/erlang/bin/erl (default false). The zip then contains only BEAM files and can be built on any OS.

prepare/1 applies these defaults unless you set them yourself: include_executables_for: [:unix], strip_beams: true, rel_templates_path pointing at Mayfly's vm.args/env.sh (no distribution, +sbwt none, RELEASE_TMP=/tmp).

Everything else is a normal release: umbrellas, several releases, config/runtime.exs, --overwrite, MIX_ENV=prod.