Use Alpine instead of Ubuntu (#59)

* Use Alpine instead of Ubuntu

* Run tag directly instead of hash

build/Dockerfile

@@ -1,6 +1,5 @@
-FROM ubuntu:latest
-RUN (apt-get update \
-     && apt-get -y install git lsb-release sudo software-properties-common)
+FROM alpine:latest
+RUN apk add --no-cache git
 RUN (git clone --recursive https://github.com/ethereum/solidity.git \
      && cd solidity \
      && git checkout 4633f3de \

build/README.md

@@ -4,7 +4,7 @@ This directory contains a dockerfile that allows you to reproduce the binary of
 To use it, first examine the Dockerfile to verify it's doing what you think it is. Then, from the main directory run:
 
     docker build --tag=lllc build
-    docker run -v $PWD:/ens a810a0a09f2b -x /ens/ENS.lll > ENS.lll.bin
+    docker run -v $PWD:/ens lllc:latest -x /ens/ENS.lll > ENS.lll.bin
     git diff
 
 If the last command shows no differences, you can be sure that the bin file represents the accurate output of the specified version of lllc.