参考教程：https://docs.docker.com/develop/develop-images/dockerfile_best-practices/
环境
- virtual box 6.1
- centos 7.8
- docker 19.03
ADD 和 COPY
Although ADD and COPY are functionally similar, generally speaking, COPY is preferred. That’s because it’s more transparent than ADD. COPY only supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support) that are not immediately obvious. Consequently, the best use for ADD is local tar file auto-extraction into the image, as in ADD rootfs.tar.xz /.
尽管 ADD and COPY 在功能上相似，但通常来说，COPY 是首选。那是因为它比 ADD 更透明。 COPY 仅支持将本地文件基本复制到容器中，而 ADD 具有一些功能（例如，仅本地 tar 提取和远程 URL 支持），这些功能并不是立即显而易见的。因此，ADD 的最佳用途是将本地 tar 文件自动提取到镜像中，如 ADD rootfs.tar.xz / 中所示。
If you have multiple Dockerfile steps that use different files from your context, COPY them individually, rather than all at once. This ensures that each step’s build cache is only invalidated (forcing the step to be re-run) if the specifically required files change.
如果您有多个 Dockerfile 步骤使用与您的上下文不同的文件，请单独复制它们，而不是一次复制所有文件。这样可以确保仅在特别需要的文件发生更改时，才使每个步骤的构建缓存无效（强制重新运行该步骤）。
For example:
例如：
COPY requirements.txt /tmp/
RUN pip install --requirement /tmp/requirements.txt
COPY . /tmp/
Results in fewer cache invalidations for the RUN step, than if you put the COPY . /tmp/ before it.
与 COPY . /tmp/ 相比，RUN 步骤导致的缓存失效更少。
Because image size matters, using ADD to fetch packages from remote URLs is strongly discouraged; you should use curl or wget instead. That way you can delete the files you no longer need after they’ve been extracted and you don’t have to add another layer in your image. For example, you should avoid doing things like:
由于镜像大小很重要，因此强烈建议不要使用 ADD 从远程 URL 获取软件包。您应该改用 curl 或 wget。这样一来，您可以删除提取后不再需要的文件，而不必在镜像中添加其他图层。例如，您应该避免做以下事情：
ADD https://example.com/big.tar.xz /usr/src/things/
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all
And instead, do something like:
相反，请执行以下操作：
RUN mkdir -p /usr/src/things
&& curl -SL https://example.com/big.tar.xz
| tar -xJC /usr/src/things
&& make -C /usr/src/things all
For other items (files, directories) that do not require ADD’s tar auto-extraction capability, you should always use COPY.
对于不需要 ADD 的 tar 自动提取功能的其他项目（文件，目录），则应始终使用e COPY。
总结
介绍了 Dockerfile 的 ADD 和 COPY 指令的最佳实践。