Build binaries from source using Docker (#14)

This change makes it easy to reproducibly build ssl-proxy for multiple platforms via Docker.

Dockerfile

@@ -0,0 +1,6 @@
+FROM golang:1.12-alpine3.9
+WORKDIR /go/src/github.com/suyashkumar/ssl-proxy
+RUN apk add --no-cache make git zip
+RUN go get -u github.com/golang/dep/cmd/dep
+COPY . .
+RUN make 

README.md

@@ -37,8 +37,17 @@ If you're using `wget`, you can fetch and uncompress the proper binary in one co
 wget -qO- $BINARY_RELEASE_LINK | tar xvz
 ```
 
-### Build from source
-You must have Golang installed on your system along with `make` and [`dep`](https://github.com/golang/dep). Then simply clone the repository and run `make`.
+### Build from source 
+#### Build from source using Docker
+You can build `ssl-proxy` for all platforms quickly using the included Docker configurations.
+
+If you have `docker-compose` installed:
+```sh
+docker-compose -f docker-compose.build.yml up
+```
+will build linux, osx, and darwin binaries (x86) and place them in a `build/` folder in your current working directory.
+#### Build from source locally
+You must have Golang installed on your system along with `make` and [`dep`](https://github.com/golang/dep). Then simply clone the repository and run `make`. 
 
 ## Attribution
 Icons made by <a href="https://www.flaticon.com/authors/those-icons" title="Those Icons">Those Icons</a> from <a href="https://www.flaticon.com/" title="Flaticon">www.flaticon.com</a> is licensed by <a href="http://creativecommons.org/licenses/by/3.0/" title="Creative Commons BY 3.0" target="_blank">CC 3.0 BY</a>

docker-compose.build.yml

@@ -0,0 +1,7 @@
+version: '2'
+services:
+  build-release:
+    build: .
+    volumes:
+      - $PWD/build:/go/src/github.com/suyashkumar/ssl-proxy/build
+    command: make release