Add dockerfile.

This makes it easy to run irccat in docker. Instructions for use
are at the top of the dockerfile

Dockerfile

@@ -0,0 +1,29 @@
+# This is a multi-part build that first statically builds irccat, and then
+# copies over the resulting binary and the default SSL root certs over into
+# a 'scratch' image, resulting in a very small image.
+#
+# You must provide a config file at /etc/irccat.[json|yaml|toml|hcl], for
+# example:
+#
+#  docker build . -t irccat
+#  docker run -d -P -v /path/to/my/config/irccat.json:/etc/irccat.json irccat
+#
+# (This will also expose the default ports: 12345 and 8045.)
+
+# Step one: fetch deps and build
+FROM golang:latest AS build
+
+ADD . /go/src/github.com/irccloud/irccat
+WORKDIR /go/src/github.com/irccloud/irccat
+
+RUN CGO_ENABLED=0 go get -t -v ./... && go build -a .
+
+# Step two: copy over the binary and root certs
+FROM scratch
+COPY --from=build /go/bin/irccat /irccat
+COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+
+EXPOSE 12345
+EXPOSE 8045
+
+CMD ["/irccat"]