Add Dockerfile for building the official CLI container

Dockerfile

@@ -0,0 +1,20 @@
+FROM golang:1.9 as builder
+
+RUN go get -u github.com/kardianos/govendor
+
+WORKDIR /go/src/mikefarah/yq
+
+COPY . /go/src/mikefarah/yq
+
+RUN govendor sync
+
+RUN CGO_ENABLED=0 go build
+
+# Choose alpine as a base image to make this useful for CI, as many
+# CI tools expect an interactive shell inside the container
+FROM alpine:3.7
+
+COPY --from=builder /go/src/mikefarah/yq/yq /usr/bin/yq
+RUN chmod +x /usr/bin/yq
+
+WORKDIR /workdir