Use bigger CI machine for docs repo

We always do a full rebuild on `elastic/docs` which takes up to 2 hours
with the standard n1-standard-4 CI machine. This PR introduces an
optimization only for builds on elastic/docs to use n2-standard-32 to
drop the build time to about 1 hour. For any other builds, we'll use
n2-standard-4 and tolerate the <20 min build time for docs given the
price difference (10x cheaper).

Author: nkammah

.buildkite/build_pr_pipeline.yml

@@ -11,6 +11,7 @@ steps:
     agents:
       provider: "gcp"
       image: family/docs-ubuntu-2204
+      machineType: ${BUILD_PR_MACHINE_TYPE}
   - key: "teardown"
     label: "teardown"
     command: |

.buildkite/hooks/pre-command

@@ -26,4 +26,10 @@ function retry {
 # https://buildkite.com/docs/pipelines/managing-log-output#redacted-environment-variables
 if [[ "$BUILDKITE_PIPELINE_SLUG" == "docs-build-pr" ]];then
     export GITHUB_TOKEN=$(retry 5 vault kv get -field=value secret/ci/elastic-docs/docs_preview_cleaner)
+    if [[ ${GITHUB_PR_BASE_REPO:="unset"} == "docs" ]]; then
+        # Docs PR require a full rebuilt - so let's boost the builds so they don't take 2 hours
+        export BUILD_PR_MACHINE_TYPE="n2-highcpu-32"
+    else
+        export BUILD_PR_MACHINE_TYPE="n2-standard-4"
+    fi
 fi

.buildkite/scripts/build_pr.sh

@@ -65,7 +65,7 @@ else
   # books for PRs to the docs repo, for now.
   # When https://github.com/elastic/docs/issues/1823 is fixed, this
   # should be removed and the original behavior restored.
-  rebuild_opt=" --rebuild"
+  rebuild_opt=" --rebuild --procs 16"
 fi