Add Docker support.

Dockerfile

@@ -0,0 +1,17 @@
+FROM golang:alpine AS build
+
+WORKDIR /go/src/github.com/yukimochi/Activity-Relay
+COPY . /go/src/github.com/yukimochi/Activity-Relay
+
+RUN  mkdir -p /rootfs/usr/bin && \
+     apk add -U --no-cache git && \
+     go get -u github.com/golang/dep/cmd/dep && \
+     dep ensure && \
+     go build -o /rootfs/usr/bin/server . && \
+     go build -o /rootfs/usr/bin/worker ./worker
+
+FROM alpine
+
+COPY --from=build /rootfs/usr/bin /usr/bin
+RUN  chmod +x /usr/bin/server /usr/bin/worker && \
+     apk add -U --no-cache ca-certificates