# 2017-08-10-dockerfile 书写注意

1. 镜像支持的层，最大数是127。
2. 不常变动的部分写在dockerfile上面，以便后续变更时可以利用缓存，减少build时间和磁盘占用。
3. 大的rpm包，如果要安装到镜像中，可以做成yum 源，yum install xx之后yum clean all，如果ADD XX.rpm /xx，这样rpm会使镜像增大。
4. RUN rm xxx 去删除上一层产生的文件不会减小镜像大小，因为包含文件的那层会一直存在。
5. 尽量不在dockerfile去修改文件权限，修改权限后的文件会生成一份新的文件导致镜像变大，修改权限在本地直接改好或写在启动脚本中。
6. 添加文件夹到指定目录，如果想把tomcat文件夹添加到home，要写成ADD tomcat/ /home/tomcat/ ， home后必须带tomcat。
7. yum install或者apt-get install 要执行clean，清除缓存，减小镜像。
8. 暴露多个端口或者设置多个环境变量或者RUN多条命令，分别写到同一层


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://misa.gitbook.io/k8s-ocp-yaml/docker-xiang-guan-wen-dang/2017-08-10-dockerfile-notes.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.