ENV command in a Dockerfile during the docker buildx build

General
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1

What’s up guys. Sorry for the stupid question but I really don’t understand how does it work.
I use buildx to build multi_arch images. See the snippet bellow.

MULTI_ARCH="linux/amd64,linux/arm/v7"
[[ -z "$NO_ARM64" ]] && MULTI_ARCH+=",linux/arm64"

echo "Base image with archs: $MULTI_ARCH"
DM_PACKAGES_URL=$DM_PACKAGES_URL docker buildx build --debug --secret id=DM_PACKAGES_URL --platform $MULTI_ARCH -t $DOCKER_REGISTRY/extender-base-env:1.4.0 -t $DOCKER_REGISTRY/extender-base-env:latest -f $SCRIPT_DIR/docker/Dockerfile.base-env $SCRIPT_DIR/docker

During the build process the Dockerfile.base-env is used. But I don’t understand how do the two ENV (lines 60,61) commands work. Please see the image below.

image
image1566×755 95.8 KB

Can you explain ?

2

It is a variable substitution: ${variable/pattern/replacement} replace the first occurrence of pattern in variable with replacement

If you are unclear about Dockerfile instructions or syntax, it is always a good idea to check the Dockerfile reference first:
https://docs.docker.com/reference/dockerfile/#environment-replacement