Properly build the base image in the pipeline

Jenkinsfile

@@ -4,7 +4,10 @@
 node('docker') {
     checkout scm
 
-    stage 'Build Containers'
+    stage 'Prepare base RVM container'
+    sh './build-base.sh'
+
+    stage 'Build Containers for Rubies'
     sh './build-rubies.sh'
 
     stage 'Publish Containers'

build-base.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+set -x
+
+source ./rubies
+
+docker build -t ${IMAGE_TAG} .

build-rubies.sh

@@ -9,7 +9,7 @@ mkdir -p work
 for RUBY in "${RUBIES[@]}"; do
     DOCKERFILE="work/dockerfile-${RUBY}"
     cat > $DOCKERFILE <<EOF
-FROM rtyler/ci-rvm:latest
+FROM ${IMAGE_TAG}:latest
 
 RUN bash -c 'source /usr/local/rvm/scripts/rvm && rvm install ${RUBY}'
 EOF