POC for publishing observability SRE images

This commit adds a step to the pull_request_pipeline buildkite definition to
push a docker image to the elastic container registry. It is added here to show
that we have the proper creds etc in CI to push the container where it needs to
go. We will likely move this into the DRA pipeline once we are confident it is
pushing to the correct place with a naming convention that works for all
consumers/producers.

The general idea is to build the container with our gradle task, then once we
have that image we can tag it with the git sha and a "latest" identifier. This
would allow consumers to choose between an exact sha for a stream like 8.19.0 or
the "latest". I will also need to factor in the case where we have the tag
*without* the sha postfix. Obviously we will want to fold this in to the existing DRA
pipeline for building/staging images but for now it seems reasonable to handle
this separately.

Author: donoghuc

.buildkite/pull_request_pipeline.yml

@@ -142,6 +142,29 @@ steps:
       docker run docker.elastic.co/logstash/logstash-observability-sre:$${QUALIFIED_VERSION} \
         logstash -e 'input { generator { count => 3 } } output { stdout { codec => rubydebug } }'
 
+  - label: "Observability SRE publishing POC"
+    key: "observability-sre-container-publishing-poc"
+    agents:
+      provider: gcp
+      imageProject: elastic-images-prod
+      image: family/platform-ingest-logstash-ubuntu-2204
+      machineType: "n2-standard-4"
+      diskSizeGb: 64
+    command: |
+      set -euo pipefail
+      source .buildkite/scripts/common/vm-agent.sh
+      source .buildkite/scripts/dra/docker-env-setup.sh
+      docker_login
+      QUALIFIED_VERSION="$(.buildkite/scripts/common/qualified-version.sh)"
+      SHA="$(git rev-parse --short HEAD)"
+      DOCKER_REPOSITORY=docker.elastic.co/logstash/logstash-observability-sre
+      ./gradlew --stacktrace artifactDockerObservabilitySRE -PfedrampHighMode=true
+      docker tag $DOCKER_REPOSITORY:$QUALIFIED_VERSION $DOCKER_REPOSITORY:$QUALIFIED_VERSION-$SHA
+      docker push $DOCKER_REPOSITORY:$QUALIFIED_VERSION-$SHA
+      docker tag $DOCKER_REPOSITORY:$QUALIFIED_VERSION $DOCKER_REPOSITORY:$QUALIFIED_VERSION-latest
+      docker push $DOCKER_REPOSITORY:$QUALIFIED_VERSION-latest
+      source .buildkite/scripts/dra/docker-env-teardown.sh
+
   - label: ":lab_coat: Integration Tests - FIPS mode / part 1-of-3"
     key: "integration-tests-fips-part-1-of-3"
     agents: