Move structure-check tools from Jenkinsfile to builder image

Jenkinsfile

@@ -85,11 +85,6 @@ pipeline {
               sh """#!/bin/bash -ex
               mkdir -p \$(dirname $GO_PROJECT) && ln -sf $WORKSPACE $GO_PROJECT
 
-              # TODO: Remove me.
-              go get github.com/segmentio/terraform-docs
-              go get github.com/s-urbaniak/terraform-examples
-              go get github.com/bronze1man/yaml2json
-
               cd $GO_PROJECT/
               make structure-check
               make bin/smoke

images/tectonic-builder/Dockerfile

@@ -22,6 +22,11 @@ FROM golang:1.8.3-stretch
 RUN go get -u github.com/golang/lint/golint
 RUN go get github.com/jstemmer/go-junit-report
 
+### Tools used by 'make structure-check'
+RUN go get github.com/segmentio/terraform-docs
+RUN go get github.com/s-urbaniak/terraform-examples
+RUN go get github.com/bronze1man/yaml2json
+
 ### License parser
 RUN go get github.com/coreos/license-bill-of-materials