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>

Dockerfile

@@ -0,0 +1,26 @@
+FROM elixir:1.10
+
+# XXX #
+# Rust is required because of the "meeseeks" dependency because it uses a Rust
+# library for some reason. I'll start looking for alternatives so we don't have
+# to worry about installing it in the future because that is not a small
+# dependency.
+
+# It's impossible to pass a command line parameter (-y) to a shell script being
+# read through stdin, so this line downloads, runs, and removes that script.
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs > /tmp/rustup.sh && \
+    chmod +x /tmp/rustup.sh && \
+    /tmp/rustup.sh -y && \
+    rm /tmp/rustup.sh
+
+# Also set the PATH to include /root/.cargo/bin, where Rust was installed to
+ENV PATH="/root/.cargo/bin:$PATH"
+
+# END XXX #
+
+RUN mkdir /app
+WORKDIR /app
+
+RUN mix local.hex --force
+
+CMD mix run --no-halt