Build packs on Fargate

With Fargate we could also potentially support Heroku style build packs, and avoid Dockerfiles / Docker builds / Docker registries.

If you just want to deploy a Ruby/Node/Go/Java/Python/Whatever app and have no special environment requirements we could just upload like we do in the Lambda platform, and pull those down in the container.

In `up.json` you'd just do something like the following for an official runtime:

```json
{
"runtime": "up/php"
}
```

Personal runtime:

```json
{
"runtime": "myorg/ruby"
}
```

This would come after #449

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Would you rather:

[](https://api.gh-polls.com/poll/01C0PWKBMDQFQF7MC9G7Q6YVC5/Buildpack%20style/vote)
[](https://api.gh-polls.com/poll/01C0PWKBMDQFQF7MC9G7Q6YVC5/Dockerfiles/vote)