Pin containers to alpine:3.4 due to problems with 3.5

Server/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:latest
+FROM alpine:3.4
 
 MAINTAINER Psiphon Inc. <info@psiphon.ca>
 LABEL Description="Alpine Linux based Psiphon Tunnel-Core Server" Vendor="Psiphon Inc." Version="1.0"

Server/Dockerfile-binary-builder

@@ -1,4 +1,4 @@
-FROM alpine:latest
+FROM alpine:3.4
 
 ENV GOLANG_VERSION 1.7.3
 ENV GOLANG_SRC_URL https://golang.org/dl/go$GOLANG_VERSION.src.tar.gz