參考教程：https://docs.docker.com/engine/reference/builder/
環境
- virtual box 6.1
- centos 7.8
- docker 19.03
ARG
ARG <name>[=<default value>]
The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag. If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning.
ARG 指令定義了一個變量，用戶可以在構建時使用 docker build 命令使用--build-arg <varname>=<value> 標志將其傳遞給構建器。如果用戶指定了未在 Dockerfile 中定義的構建參數，則構建會輸出警告。
[Warning] One or more build-args [foo] were not consumed.
A Dockerfile may include one or more ARG instructions. For example, the following is a valid Dockerfile:
Dockerfile 可能包含一個或多個 ARG 指令。例如，以下是有效的 Dockerfile：
FROM busybox
ARG user1
ARG buildno
# ...
Warning:
It is not recommended to use build-time variables for passing secrets like github keys, user credentials etc. Build-time variable values are visible to any user of the image with the
docker historycommand.
警告：
不建議使用構建時變量來傳遞諸如 github 密鑰，用戶憑據等機密。構建時變量值對於使用
docker history命令的鏡像的任何用戶都是可見的。
默認值
An ARG instruction can optionally include a default value:
ARG 指令可以選擇包含默認值：
FROM busybox
ARG user1=someuser
ARG buildno=1
# ...
If an ARG instruction has a default value and if there is no value passed at build-time, the builder uses the default.
如果 ARG 指令具有默認值，並且在構建時未傳遞任何值，則構建器將使用默認值。
范圍
An ARG variable definition comes into effect from the line on which it is defined in the Dockerfile not from the argument’s use on the command-line or elsewhere. For example, consider this Dockerfile:
ARG 變量從 Dockerfile 中定義的行開始生效，而不是從命令行或其他地方的自變量使用開始。例如，考慮以下 Dockerfile：
FROM busybox
USER ${user:-some_user}
ARG user
USER $user
# ...
A user builds this file by calling:
$ docker build --build-arg user=what_user .
The USER at line 2 evaluates to some_user as the user variable is defined on the subsequent line 3. The USER at line 4 evaluates to what_user as user is defined and the what_user value was passed on the command line. Prior to its definition by an ARG instruction, any use of a variable results in an empty string.
第 2 行的 USER 評估為 some_user，因為在隨后的第 3 行中定義了 USER 變量。第 4 行的 USER 評估為 what_user，因為定義了 user，並且為 what_user 在命令行中傳遞。在通過 ARG 指令對其進行定義之前，對變量的任何使用都會導致一個空字符串。
An ARG instruction goes out of scope at the end of the build stage where it was defined. To use an arg in multiple stages, each stage must include the ARG instruction.
ARG 指令在定義它的構建階段結束時超出范圍。要在多個階段使用變量，每個階段都必須包含 ARG 指令。
FROM busybox
ARG SETTINGS
RUN ./run/setup $SETTINGS
FROM busybox
ARG SETTINGS
RUN ./run/other $SETTINGS
總結
介紹了 Dockerfile 中 ARG 指令的說明，默認值和范圍。