--build-args/ARG does not overwrite ENV variables of the same name from a base image #3344
Description
Description
ARG variables of the same name as an environment variable set in a base image do not overwrite the value of the variable during build time. Is it intentional for the primary environment to take precedence over the temporary environment? This is especially irritating when a base image sets a proxy value to empty and the child image needs to temporarily set a build environment that uses a different proxy value than the parent image.
Steps to reproduce the issue:
- Build a parent image and set a variable with ENV
- Base a child image off the built parent and apply an ARG of the same name as the parent ENV var.
- Echo the ARG in a dockerfile RUN directive in the child
- Build the child. Note that the ENV variable of the parent is echo'd not the temporary ARG value
Describe the results you received:
Create parent dockerfile and set an ENV variable.
# parent.dockerfile
FROM alpine:latest
ENV HELLO="World"Build the parent image
❯ docker build -t parent:latest -f parent.dockerfile .Create a child dockerfile and set a build-arg with the same name as the ENV var from parent.
# child.dockerfile
FROM parent:latest
ARG HELLO
RUN echo "HELLO=${HELLO}"Build the child image.
❯ docker build -t child:latest --build-arg HELLO=Earth -f child.dockerfile --no-cache .
Sending build context to Docker daemon 3.072kB
Step 1/4 : FROM parent:latest
---> b319a2900116
Step 2/4 : ARG HELLO
---> Running in df26b87ef593
Removing intermediate container df26b87ef593
---> 037665231ae3
Step 3/4 : RUN echo "HELLO=${HELLO}"
---> Running in b79b3944af01
HELLO=World
Removing intermediate container b79b3944af01
---> 6772fb18124d
Step 4/4 : CMD [ "echo", "HELLO=${HELLO}" ]
---> Running in 8f7fc5bc845b
Removing intermediate container 8f7fc5bc845b
---> fdaed33c1026
Successfully built fdaed33c1026
Successfully tagged child:latestResult: In Step 3/4 the value echo'd is that of the original ENV var in the parent.
Describe the results you expected:
I would expect that the temporary environment would take precedence over the core environment during build and echo would produce HELLO=Earth in the example above.
Additional information you deem important (e.g. issue happens only occasionally):
Output of docker version:
Client:
Cloud integration: 1.0.17
Version: 20.10.8
API version: 1.41
Go version: go1.16.6
Git commit: 3967b7d
Built: Fri Jul 30 19:55:20 2021
OS/Arch: darwin/amd64
Context: default
Experimental: true
Server: Docker Engine - Community
Engine:
Version: 20.10.8
API version: 1.41 (minimum version 1.12)
Go version: go1.16.6
Git commit: 75249d8
Built: Fri Jul 30 19:52:10 2021
OS/Arch: linux/amd64
Experimental: false
containerd:
Version: 1.4.9
GitCommit: e25210fe30a0a703442421b0f60afac609f950a3
runc:
Version: 1.0.1
GitCommit: v1.0.1-0-g4144b63
docker-init:
Version: 0.19.0
GitCommit: de40ad0
Output of docker info:
Client:
Context: default
Debug Mode: false
Plugins:
buildx: Build with BuildKit (Docker Inc., v0.6.1-docker)
compose: Docker Compose (Docker Inc., v2.0.0-rc.3)
scan: Docker Scan (Docker Inc., v0.8.0)
Server:
Containers: 6
Running: 3
Paused: 0
Stopped: 3
Images: 67
Server Version: 20.10.8
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Native Overlay Diff: true
userxattr: false
Logging Driver: json-file
Cgroup Driver: cgroupfs
Cgroup Version: 1
Plugins:
Volume: local
Network: bridge host ipvlan macvlan null overlay
Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
Swarm: inactive
Runtimes: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
Default Runtime: runc
Init Binary: docker-init
containerd version: e25210fe30a0a703442421b0f60afac609f950a3
runc version: v1.0.1-0-g4144b63
init version: de40ad0
Security Options:
seccomp
Profile: default
Kernel Version: 5.10.47-linuxkit
Operating System: Docker Desktop
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 7.774GiB
Name: docker-desktop
ID: BSL6:V5BU:UHMQ:5ZQ7:QELY:VD6D:A7U5:434O:7WSO:FHHJ:SAMI:UONQ
Docker Root Dir: /var/lib/docker
Debug Mode: false
HTTP Proxy: http://proxy-lmi.global.lmco.com:80
HTTPS Proxy: http://proxy-lmi.global.lmco.com:80
No Proxy: 127.0.0.10/8,localhost,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16,.lmco.com
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false
Additional environment details (AWS, VirtualBox, physical, etc.):