ci: add .dockerignore to exclude env files to prevent potential key leakages 避免潜在的密钥泄漏

[KevenLi8888]

What changes are made 变更的内容
Add a .dockerignore file to exclude the env files when building the image locally.
添加了.dockerignore文件，以在本地构建镜像时排除环境变量相关文件。

Why does it matter 变更的原因
When developing and building the docker image locally, if a .env file is used to specify the API key and access code, it will be included in the image that is built locally, thus leads to a secret key leak when the image is pushed to a public docker registry.
(This week we encountered an OpenAI API key leakage. Upon investigation, it was found to be caused by the inclusion of the .env file in the image.)
在本地开发和构建Docker镜像时，如果使用.env文件来指定API密钥和密码，并在本地构建镜像，.env文件将会包含在构建的镜像中。当该镜像被推到公开的Docker Registry时，会导致密钥泄露。
（本周遇到了OpenAI API密钥泄露的问题，经排查发现为镜像中包含.env文件所致）

How the .env file is included into the image

COPY . .
# .env file is copied from the codebase into the image working directory
RUN yarn build
# yarn build includes the .env file in /app/.next/standalone
......
COPY --from=builder /app/.next/standalone ./
# .env file is copied into the base working directory in the image

The file structure inside the image

$ docker exec -it chatgpt-next-web sh
/app # ls -a
.  ..   .env  .next  node_modules  package.json  public  server.js