Update Dockerfile and README

Dockerfile

@@ -1,7 +1,11 @@
 FROM golang:alpine as build
 WORKDIR /go/src/github.com/ssllabs/ssllabs-scan
 COPY . .
-RUN apk --no-cache add git \
+RUN apk update \
+  && apk upgrade \
+  && apk add --no-cache ca-certificates \
+  && update-ca-certificates 2>/dev/null || true
+  && apk --no-cache add git \
   && go get -u github.com/ssllabs/ssllabs-scan/...
 
 FROM alpine:latest

README.md

@@ -21,6 +21,44 @@ https://www.ssllabs.com/about/terms.html
 
 * Go >= 1.3
 
+## Installation
+
+# Prebuilt Binaries
+
+A [precompiled version is available](https://github.com/ssllabs/ssllabs-scan/releases) with each release.
+
+# Using Docker
+
+1. Build the [Docker](https://docs.docker.com/) image:
+
+```bash
+sudo docker build -t ssllabs-scan https://github.com/ssllabs/ssllabs-scan.git
+```
+
+2. Run the Docker image:
+
+```bash
+sudo docker run ssllabs-scan example.com
+```
+
+# From Source
+
+If you prefer to build your own binary from the latest release of the source code, make sure you have a correctly configured **Go >= 1.10** environment. More information about how to achieve this can be found [on the golang website.](https://golang.org/doc/install) Then, take the following steps:
+
+1. Download ssllabs-scan:
+
+```bash
+go get -u github.com/ssllabs/ssllabs-scan/...
+```
+
+2. If you wish to rebuild the binaries from the source code:
+
+```bash
+cd $GOPATH/src/github.com/ssllabs/ssllabs-scan
+
+go install ./...
+```
+
 ## Usage 
 
 SYNOPSIS