Fail cloude2e step if tests can't run, fix image publication (elastic#5000)

Fail the pipeline if cloude2e tests can't run.
mage docker:image now uses the DOCKER_IMAGE env var.

Author: michel-laterman

.buildkite/scripts/cloud_e2e_test.sh

@@ -41,7 +41,7 @@ if [[ "${FLEET_SERVER_URL}" == "" ]]; then
             --style "error"
     fi
     echo "${message}"
-    exit 0
+    exit 1
 fi
 
 echo "--- Trigger cloud E2E test"

magefile.go

@@ -966,6 +966,7 @@ func (Docker) Cover() error {
 // DEV creates a development image.
 // SNAPSHOT creates a snapshot image.
 // VERSION_QUALIFIER may be used to manually specify a version qualifer for the image tag.
+// DOCKER_IMAGE may be used to completely specify the image name.
 // DOCKER_IMAGE_TAG may be used to completely specify the image tag.
 func (Docker) Image() error {
 	dockerFile := "Dockerfile"
@@ -980,6 +981,9 @@ func (Docker) Image() error {
 		dockerFile = dockerBuilderFIPS
 		image += "-fips"
 	}
+	if v, ok := os.LookupEnv(envDockerImage); ok && v != "" {
+		image = v
+	}
 
 	return sh.RunWithV(map[string]string{"DOCKER_BUILDKIT": "1"}, "docker", "build",
 		"--build-arg", "GO_VERSION="+getGoVersion(),
@@ -1602,7 +1606,6 @@ func (Test) Benchstat() error {
 func (Test) E2e() {
 	os.Setenv(envDev, "true")
 	os.Setenv(envSnapshot, "true")
-	os.Setenv(envDockerImage, "fleet-server-e2e-agent")
 
 	// Set PLATFORMS to linux/$GOARCH + a binary for the local system
 	pList := []string{