Pin Alpine terraform version

We pin the version to guarantee some stability but we don't test
for that version in specs to avoid to maintain the version in
2 places.

terraform/Dockerfile

@@ -1,4 +1,5 @@
-FROM alpine
+FROM alpine:3.3
+
 ENV PATH $PATH:/usr/local/bin
 ENV TERRAFORM_VER 0.6.13
 ENV TERRAFORM_ZIP terraform_${TERRAFORM_VER}_linux_amd64.zip

terraform/terraform_spec.rb

@@ -19,8 +19,8 @@
     set :docker_image, find_image_id('terraform:latest')
   }
 
-  it "installs the Alpine linux" do
-    expect(file("/etc/alpine-release")).to be_file
+  it "installs Alpine" do
+    expect(command("cat /etc/issue | head -1").stdout).to include("Alpine Linux")
   end
 
   it "installs Root Certificates" do