Fix built image id output for subsequent builds

For the first `docker build` task in a job, DOCKER_TASK_BUILT_IMAGES is
correct. But for subsequent builds, instead of appending the new image's
hash it was duplicating the list, e.g.

- after first build: firstBuildId
- after second build: firstBuildId;firstBuildId
- after third build: firstBuildId;firstBuildId;firstBuildId;firstBuildId
- etc.

Instead we append the new image hash, as was seemingly the original
intention. So now:

- after first build: firstBuildId
- after second build: firstBuildId;secondBuildId
- after third build: firstBuildId;secondBuildId;thirdBuildId
- etc.

common-npm-packages/docker-common/containerimageutils.ts

@@ -206,7 +206,7 @@ export function shareBuiltImageId(builtImageId: string) {
     let builtImages: string = tl.getVariable("DOCKER_TASK_BUILT_IMAGES");
 
     if (builtImages && builtImages != "") {
-        const newImageId = `${IMAGE_SEPARATOR_CHAR}${builtImages}`;
+        const newImageId = `${IMAGE_SEPARATOR_CHAR}${builtImageId}`;
 
         if (newImageId.length + builtImages.length > ENV_VARIABLE_MAX_SIZE) {
             tl.debug("Images id truncated maximum environment variable size reached.");

common-npm-packages/docker-common/package.json

@@ -1,6 +1,6 @@
 {
     "name": "azure-pipelines-tasks-docker-common",
-    "version": "2.226.0",
+    "version": "2.226.1",
     "description": "Common Library for Azure Rest Calls",
     "repository": {
         "type": "git",