参考教程：https://docs.docker.com/engine/reference/builder/

环境

1. virtual box 6.1
2. centos 7.8
3. docker 19.03

FROM

FROM [--platform=<platform>] <image> [AS <name>]

Or

FROM [--platform=<platform>] <image>[:<tag>] [AS <name>]

Or

FROM [--platform=<platform>] <image>[@<digest>] [AS <name>]

The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions. As such, a valid Dockerfile must start with a FROM instruction. The image can be any valid image – it is especially easy to start by pulling an image from the Public Repositories.

FROM 指令初始化一个新的构建阶段，并为后续指令设置基础镜像。因此，有效的 Dockerfile 必须以 FROM 指令开头。该镜像可以是任何有效的镜像–很容易通过从Public Repositories 拉取一个镜像。

• ARG is the only instruction that may precede FROM in the Dockerfile. See Understand how ARG and FROM interact.

• ARG 是唯一可以在 Dockerfile 中的 FROM 之前的指令。请参阅了解ARG和FROM之间的交互方式。

• FROM can appear multiple times within a single Dockerfile to create multiple images or use one build stage as a dependency for another. Simply make a note of the last image ID output by the commit before each new FROM instruction. Each FROM instruction clears any state created by previous instructions.

• FROM 可以在单个 Dockerfile 中多次出现，以创建多个镜像或将一个构建阶段用作对另一构建阶段的依赖。只需在每条新的 FROM 指令之前记录一次提交输出的最后一个镜像 ID。每条 FROM 指令清除由先前指令创建的任何状态。

• Optionally a name can be given to a new build stage by adding AS name to the FROM instruction. The name can be used in subsequent FROM and COPY --from=<name> instructions to refer to the image built in this stage.

• 可以选择在 FROM 指令中添加 AS 名称，从而为新的构建阶段指定名称。该名称可以在后续的 FROM 和 COPY --from = <name> 指令中使用，以引用此阶段构建的镜像。

• The tag or digest values are optional. If you omit either of them, the builder assumes a latest tag by default. The builder returns an error if it cannot find the tag value.

• tag 或 digest 值是可选的。如果您忽略其中任何一个，则默认情况下，构建器将采用 latest 标签。如果构建器找不到 tag 值，则返回错误。

The optional --platform flag can be used to specify the platform of the image in case FROM references a multi-platform image. For example, linux/amd64, linux/arm64, or windows/amd64. By default, the target platform of the build request is used. Global build arguments can be used in the value of this flag, for example automatic platform ARGs allow you to force a stage to native build platform (--platform=$BUILDPLATFORM), and use it to cross-compile to the target platform inside the stage.

可选的 --platform 标志可用于指定镜像的平台，以防万一 ·FROM· 引用了多平台镜像。例如，linux/amd64，linux/arm64 或 windows/amd64。默认情况下，使用构建请求的目标平台。可以在此标志的值中使用全局构建参数，例如 automatic platform ARGs 允许您将阶段强制为本机构建平台（--platform = $BUILDPLATFORM），并使用该平台将其交叉编译到阶段内部的目标平台。

Understand how ARG and FROM interac

FROM instructions support variables that are declared by any ARG instructions that occur before the first FROM.

FROM 指令支持由出现在第一个 FROM 之前的任何 ARG 指令声明的变量。

ARG  CODE_VERSION=latest
FROM base:${CODE_VERSION}
CMD  /code/run-app

FROM extras:${CODE_VERSION}
CMD  /code/run-extras

An ARG declared before a FROM is outside of a build stage, so it can’t be used in any instruction after a FROM. To use the default value of an ARG declared before the first FROM use an ARG instruction without a value inside of a build stage:

在 FROM 之前声明的 ARG 在构建阶段之外，因此不能在 FROM 之后的任何指令中使用。要使用在第一个 FROM 之前声明的 ARG 的默认值，请使用在构建阶段内部不带值的 ARG 指令：

ARG VERSION=latest
FROM busybox:$VERSION
ARG VERSION
RUN echo $VERSION > image_version

总结

介绍了 Dockerfile 中 FROM 指令的使用。