参考教程：https://docs.docker.com/engine/reference/builder/
环境
- virtual box 6.1
- centos 7.8
- docker 19.03
HEALTHCHECK
The HEALTHCHECK instruction has two forms:
HEALTHCHECK 指令有两种形式：
HEALTHCHECK [OPTIONS] CMD command(check container health by running a command inside the container)HEALTHCHECK [OPTIONS] CMD command（通过在容器内部运行命令来检查容器的运行状况）HEALTHCHECK NONE(disable any healthcheck inherited from the base image)HEALTHCHECK NONE（禁用从基础镜像继承的任何健康检查）
The HEALTHCHECK instruction tells Docker how to test a container to check that it is still working. This can detect cases such as a web server that is stuck in an infinite loop and unable to handle new connections, even though the server process is still running.
HEALTHCHECK 指令告诉 Docker 如何测试容器以检查其是否仍在工作。这样可以检测到诸如 Web 服务器陷入无限循环并且无法处理新连接的情况，即使服务器进程仍在运行。
When a container has a healthcheck specified, it has a health status in addition to its normal status. This status is initially starting. Whenever a health check passes, it becomes healthy (whatever state it was previously in). After a certain number of consecutive failures, it becomes unhealthy.
指定容器的运行状况检查后，除了其正常状态外，容器还具有 health status。此状态最初是 starting。只要健康检查通过，它就会变成 healthy 状态（无论以前处于什么状态）。在一定数量的连续故障之后，它变得 unhealthy。
The options that can appear before CMD are:
可以在 CMD 之前出现的选项是：
--interval=DURATION(default:30s)--timeout=DURATION(default:30s)--start-period=DURATION(default:0s)--retries=N(default:3)
The health check will first run interval seconds after the container is started, and then again interval seconds after each previous check completes.
运行状况检查将在容器启动后首先运行 interval 秒，然后在之前每次检查完成后再次 interval秒。
If a single run of the check takes longer than timeout seconds then the check is considered to have failed.
如果单次检查花费的时间超过 timeout 秒，则认为检查失败。
It takes retries consecutive failures of the health check for the container to be considered unhealthy.
要使容器被视为 unhealthy，需要进行 retries 连续失败的健康检查。
start period provides initialization time for containers that need time to bootstrap. Probe failure during that period will not be counted towards the maximum number of retries. However, if a health check succeeds during the start period, the container is considered started and all consecutive failures will be counted towards the maximum number of retries.
开始时间 为需要时间进行引导的容器提供了初始化时间。在此期间内的探针故障将不计入最大重试次数。但是，如果运行状况检查在启动期间成功，则认为该容器已启动，并且所有连续失败将计入最大重试次数。
There can only be one HEALTHCHECK instruction in a Dockerfile. If you list more than one then only the last HEALTHCHECK will take effect.
Dockerfile 中只能有一条 HEALTHCHECK 指令。如果您列出多个，则只有最后一个 HEALTHCHECK 才会生效。
The command after the CMD keyword can be either a shell command (e.g. HEALTHCHECK CMD /bin/check-running) or an exec array (as with other Dockerfile commands; see e.g. ENTRYPOINT for details).
关键字 CMD 之后的命令可以是 shell 命令（例如 HEALTHCHECK CMD /bin/check-running）或 exec 数组（与其他 Dockerfile 命令一样；有关详细信息，请参见 ENTRYPOINT）。
The command’s exit status indicates the health status of the container. The possible values are:
命令的退出状态指示容器的健康状态。可能的值为：
- 0: success - the container is healthy and ready for use
- 1: unhealthy - the container is not working correctly
- 2: reserved - do not use this exit code
- 0：成功-容器健康且可以使用
- 1：不健康-容器无法正常工作
- 2：保留-请勿使用此退出代码
For example, to check every five minutes or so that a web-server is able to serve the site’s main page within three seconds:
例如，要每五分钟检查一次，以便网络服务器能够在三秒钟内为网站的首页提供服务：
HEALTHCHECK --interval=5m --timeout=3s
CMD curl -f http://localhost/ || exit 1
To help debug failing probes, any output text (UTF-8 encoded) that the command writes on stdout or stderr will be stored in the health status and can be queried with docker inspect. Such output should be kept short (only the first 4096 bytes are stored currently).
为了帮助调试失败的探针，该命令在 stdout 或 stderr 上写入的任何输出文本（UTF-8编码）将以健康状态存储，并可以通过 docker inspect 查询。此类输出应保持简短（当前仅存储前4096个字节）。
When the health status of a container changes, a health_status event is generated with the new status.
当容器的健康状态发生变化时，将使用新状态生成一个 health_status 事件。
总结
介绍了 Dockerfile 中 HEALTHCHECK 指令的用法和注意事项。