Add Dockerfile

This enables users to build, ship and run lego with Docker.

Dockerfile

@@ -0,0 +1,17 @@
+FROM alpine:3.3
+
+ENV GOPATH /go
+
+RUN apk update && apk add ca-certificates go git && \
+    rm -rf /var/cache/apk/*
+
+COPY . /go/src/github.com/xenolf/lego
+
+RUN cd /go/src/github.com/xenolf/lego && \
+	go get ./... && \
+	go build -o /usr/bin/lego . && \
+	apk del ca-certificates go git && \
+	rm -rf /var/cache/apk/* && \
+	rm -rf /go
+
+ENTRYPOINT [ "/usr/bin/lego" ]

README.md

@@ -19,6 +19,11 @@ To install from source, just run
 go get -u github.com/xenolf/lego
 ```
 
+To build lego inside a Docker container, just run
+```
+docker build -t lego .
+```
+
 #### Features
 
 - Register with CA