add instructions for alpine

**1. Please specify the following:**
- OS: [e.g. OS X, Ubuntu, Windows, etc.]: alpine container
- Shell (Bash, PowerShell, Zsh, etc.): sh

- [x] **I have reviewed the troubleshooting tips described [here](https://github.com/aws/aws-elastic-beanstalk-cli-setup#23-troubleshooting) and they do not solve my problem**

**2. Description**
Please describe the problem you are facing as precisely as possible in this section

Can we add prerequisites for alpine? These appear to work

```
docker run -it alpine:3.11.5
apk add build-base python3-dev libffi-dev openssl-dev
```

Start from the installation instructions and the linked troubleshooting section, then review how platform prerequisites are documented. Verify the listed apk packages in an Alpine container; done means Alpine users can find accurate prerequisite instructions for installing the EB CLI.

Written by the indexing model from the issue text.