Adding Alpine Based Images and Restructuring the repo

[RudolfMan]

• added Dockerfiles for avvo/alpine:3.9 based erlang and elixir images. And to differentiate between ubuntu and alpine - I put them into folders with respective names.

  So for example elixir/1.6.2/Dockerfile --moved to--> elixir/1.6.2/ubuntu/Dockerfile.

• folders with Dockerfiles from ./elixir-release/ are moved to ./elixir-release/alpine/ and
  Dockerfiles from ./elixir-release-ubuntu/ are moved to ./elixir-release/ubuntu/

• modified push.sh script respectively to tag images with version and OS name
  e.g. avvo/elixir:1.9.2-alpine and avvo/elixir:latest-alpine

• removed the outdated ./elixir-circle/ at all.

Now structure looks like

.
...
├── elixir
│   └── 1.9
│       ├── alpine
│       │   └── Dockerfile
│       └── ubuntu
│           └── Dockerfile
├── elixir-release
│   ├── alpine
│   │   ├── 3.6
│   │   │   └── Dockerfile
│   │   └── 3.9
│   │       └── Dockerfile
│   └── ubuntu
│       ├── 18.04
│       │   └── Dockerfile
│       └── xenial
│           └── Dockerfile
├── erlang
│   └── 22
│       ├── alpine
│       │   └── Dockerfile
│       └── ubuntu
│           └── Dockerfile
└── push.sh

Plus added example of multistage Docker build for prod to README.md