Add Dockerfile, docker-compose.yml, and docker.env

Omnibot can be run using Docker and docker-compose using the provided
files.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

docker-compose.yml

@@ -0,0 +1,13 @@
+version: "3.8"
+services:
+  omnibot:
+    #image: "elixir:1.10"
+    build: .
+    #environment:
+    #MIX_ENV: prod
+    env_file: docker.env
+    volumes:
+      - ".:/app"
+    working_dir: "/app"
+    restart: always
+    #entrypoint: "mix local.hex --force; mix run --no-halt"