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man/docker-build: fix/expand --build-arg

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Explain that variable value is optional and is taken from environment
if not set.

Clarify the meaning of ARG, and mention that ENV overrides ARG of the
same name.

Move example to the end, and refer to the Dockerfile and buildx docs
for more details (replacing the obsolete link).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
This commit is contained in:
Kir Kolyshkin
2026-10-19 01:15:00 -07:00
parent 52a2671034
commit 72f6e50c62
1 changed files with 14 additions and 6 deletions
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man/docker-build.1.md
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@@ -83,16 +83,24 @@ set as the **URL**, the repository is cloned locally and then sent as the contex
Add a line to /etc/hosts. The format is hostname=ip, or hostname:ip.
The **--add-host** option can be set multiple times.
**--build-arg** *variable*
name and value of a **buildarg**.
**--build-arg** *name*[=*value*]
Set a build-time variable. The *name* is the variable name, as declared
by an `ARG` instruction in the Dockerfile. If *value* is omitted, the
value of the variable *name* from the local environment is used; if it is
not set there either, the default from the `ARG` instruction (if any) is
used.
Build-time variables are available as environment variables to commands
run by the Dockerfile's `RUN` instructions, and can be used for variable
expansion in other Dockerfile instructions. An environment variable defined
by an `ENV` instruction always overrides an `ARG` of the same name. This
mechanism is not meant for passing secret values.
For example, if you want to pass a value for `http_proxy`, use
`--build-arg=http_proxy="http://some.proxy.url"`
Users pass these values at build-time. Docker uses the `buildargs` as the
environment context for command(s) run via the Dockerfile's `RUN` instruction
or for variable expansion in other Dockerfile instructions. This is not meant
for passing secret values. [Read more about the buildargs instruction](https://docs.docker.com/engine/reference/builder/#arg)
See also https://docs.docker.com/reference/dockerfile/#arg and
https://docs.docker.com/reference/cli/docker/buildx/build/#build-arg.
**--cache-from** ""
Set image that will be used as a build cache source.