Dockerfile: document ALPINE_VERSION build-arg

[thaJeztah]

• relates to Dockerfile: bump gotest.tools/gotestsum v1.12.3 (for go1.25) #6179 (comment)

Dockerfile: document ALPINE_VERSION build-arg

docker build --call outline .

TARGET: binary

BUILD ARG               VALUE    DESCRIPTION
BASE_VARIANT            alpine
ALPINE_VERSION          3.21     sets the version of the alpine base image to use, including for the golang image.
GO_VERSION              1.24.5
XX_VERSION              1.6.1
GOVERSIONINFO_VERSION   v1.4.1
GO_LINKMODE             static   defines if static or dynamic binary should be produced
GO_BUILDTAGS                     defines additional build tags
GO_STRIP                         strips debugging symbols if set
CGO_ENABLED                      manually sets if cgo is used
VERSION                          sets the version for the produced binary
PACKAGER_NAME                    sets the company that produced the windows binary

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