simplify logic for skipping docker build

fixes: CCI-285
flag = none

Test-Plan:
- run with skip-docker-build and ensure it works.
- run without and ensure it works

Change-Id: Ida7d623b7b20496dc9d5cfeb6c571c56eca49b7e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/230129
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: James Butters <jbutters@instructure.com>
Reviewed-by: Derek Bender <djbender@instructure.com>
Reviewed-by: Jacob Powell <spowell@instructure.com>
QA-Review: Jacob Powell <spowell@instructure.com>
Product-Review: Jacob Powell <spowell@instructure.com>

Jenkinsfile

@@ -38,16 +38,6 @@ def getImageTagVersion() {
   flags.getImageTagVersion()
 }
 
-def runBuildImageMaybe(block) {
-  def flags = load('build/new-jenkins/groovy/commit-flags.groovy')
-  if (flags.hasFlag('skip-docker-build')) {
-    echo "Skip building image requested"
-  } else {
-    def successes = load('build/new-jenkins/groovy/successes.groovy')
-    successes.skipIfPreviouslySuccessful("build-and-push-image", true, block)
-  }
-}
-
 def skipIfPreviouslySuccessful(name, block) {
   def successes = load('build/new-jenkins/groovy/successes.groovy')
   successes.skipIfPreviouslySuccessful(name, true, block)
@@ -206,21 +196,26 @@ pipeline {
     stage ('Build Docker Image') {
       steps {
         timeout(time: 36) { /* this timeout is `2 * average build time` which currently: 18m * 2 = 36m */
-          runBuildImageMaybe() {
+          skipIfPreviouslySuccessful('docker-build-and-push') {
             script {
               def flags = load('build/new-jenkins/groovy/commit-flags.groovy')
-              if (!flags.hasFlag('skip-cache')) {
-                // canvas-lms:$GERRIT_BRANCH as the image cache for this build (i.e. canvas-lms:master)
-                sh 'docker pull $MERGE_TAG || true'
+              if (flags.hasFlag('skip-docker-build')) {
+                sh 'docker pull $MERGE_TAG'
+                sh 'docker tag $MERGE_TAG $PATCHSET_TAG'
+              }
+              else {
+                if (!flags.hasFlag('skip-cache')) {
+                  sh 'docker pull $MERGE_TAG || true'
+                }
+                sh """
+                  docker build \
+                    --tag $PATCHSET_TAG \
+                    --build-arg RUBY_PASSENGER=$RUBY_PASSENGER \
+                    --build-arg POSTGRES_VERSION=$POSTGRES \
+                    .
+                """
               }
             }
-            sh """
-              docker build \
-                --tag $PATCHSET_TAG \
-                --build-arg RUBY_PASSENGER=$RUBY_PASSENGER \
-                --build-arg POSTGRES_VERSION=$POSTGRES \
-                .
-            """
             sh "docker push $PATCHSET_TAG"
           }
         }

build/new-jenkins/groovy/commit-flags.groovy

@@ -39,7 +39,7 @@ def hasFlag(name) {
 
 def getImageTagVersion() {
   // 'refs/changes/63/181863/8' -> '63.181863.8'
-  return hasFlag('skip-docker-build') ? 'master' : "${env.GERRIT_REFSPEC}".minus('refs/changes/').replaceAll('/','.')
+  return "${env.GERRIT_REFSPEC}".minus('refs/changes/').replaceAll('/','.')
 }
 
 def forceRunCoverage() {