fix(ci): list plugin.sh in build.by so the image build can find it

CNB only puts the files named in `build.by` into the build context of a
`docker.build` environment image, and silently omits everything else
rather than failing. The kaniko image's `COPY .ci/kaniko/plugin.sh` was
therefore reading from an empty "client dir", which is why the build
failed with `"/.ci/kaniko/plugin.sh": not found` while the file is very
much in the repository.

The Dockerfile itself is exempt from the filter, so `dockerfile:` alone
had looked sufficient.

Ref: #16

.cnb.yml

@@ -112,6 +112,10 @@ main:
       docker:
         build:
           dockerfile: .ci/kaniko/Dockerfile
+          # CNB 只把 `by` 里列出的文件放进构建上下文，`plugin.sh` 不在列的话
+          # 并不会报错，只是 Dockerfile 里那句 COPY 找不到文件。
+          by:
+            - .ci/kaniko/plugin.sh
       stages:
         - name: login
           script: |