ARG vs ENV Precedence
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area/builder
kind/bug
- Dominant language
- Go
- Stars
- 6.1k
- Forks
- 2.2k
- Avg merge
- 1d 15h
- Merged PRs (30d)
- 43
Description
Description
I apologize if this is the wrong place to post about this issue. Please point me to the right place if this is so.
When the base image has an environment variable and the same environment variable is being passed as a build argument but with a different value, the precedence order isn't consistent. On AWS linux, the image environment variable takes precedence. On my local Mac, the build argument takes precedence. I think the Mac behavior is the correct one since the build argument is more recent. But they should be consistent whatever the correct choice is.
Describe the results you expected:
I expected both to use the build argument.
Steps to reproduce the issue:
- Build a base image with any environment variable.
docker build -t base_image . -f base.Dockerfile - Use the base image to build a new image and pass it a build argument with a different value.
docker build -t arg_image --build-arg VAR=val2 . -f arg.Dockerfile
Files
base.Dockerfile
FROM python:3.7.9
ENV VAR=val1
arg.Dockerfile
FROM base_image
ARG VAR
RUN $VAR
Linux
Output of docker version:
Client: Docker Engine - Community
Version: 20.10.1
API version: 1.40
Go version: go1.13.15
Git commit: 831ebea
Built: Tue Dec 15 04:34:58 2020
OS/Arch: linux/amd64
Context: default
Experimental: true
Server: Docker Engine - Community
Engine:
Version: 19.03.13
API version: 1.40 (minimum version 1.12)
Go version: go1.13.15
Git commit: 4484c46d9d
Built: Wed Sep 16 17:01:20 2020
OS/Arch: linux/amd64
Experimental: false
containerd:
Version: 1.4.3
GitCommit: 269548fa27e0089a8b8278fc4fc781d7f65a939b
runc:
Version: 1.0.0-rc92
GitCommit: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
docker-init:
Version: 0.18.0
GitCommit: fec3683
Output of docker info:
Client:
Context: default
Debug Mode: false
Plugins:
app: Docker App (Docker Inc., v0.9.1-beta3)
buildx: Build with BuildKit (Docker Inc., v0.5.0-docker)
Server:
Containers: 0
Running: 0
Paused: 0
Stopped: 0
Images: 295
Server Version: 19.03.13
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
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: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 269548fa27e0089a8b8278fc4fc781d7f65a939b
runc version: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
init version: fec3683
Security Options:
apparmor
seccomp
Profile: default
Kernel Version: 5.4.0-1030-aws
Operating System: Ubuntu 20.04.1 LTS
OSType: linux
Architecture: x86_64
CPUs: 2
Total Memory: 7.698GiB
Name: ip-10-1-6-74
ID: TQCU:3JXP:WGDF:KATD:3FAS:MZIY:IXAL:P3D6:BMD5:K3VV:CYPX:P4KT
Docker Root Dir: /var/lib/docker
Debug Mode: false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false
WARNING: No swap limit support
Output from steps
$ docker build -t base_image . -f base.Dockerfile
Sending build context to Docker daemon 3.072kB
Step 1/2 : FROM python:3.7.9
---> 7fefbebd95b5
Step 2/2 : ENV VAR=val1
---> Using cache
---> 802e36d3b625
Successfully built 802e36d3b625
Successfully tagged base_image:latest
$ docker build -t arg_image --build-arg VAR=val2 . -f arg.Dockerfile
Sending build context to Docker daemon 3.072kB
Step 1/3 : FROM base_image
---> 802e36d3b625
Step 2/3 : ARG VAR
---> Running in d0d13d54d20d
Removing intermediate container d0d13d54d20d
---> 1b00b76e968a
Step 3/3 : RUN $VAR
---> Running in 03e5e4bb6421
/bin/sh: 1: val1: not found
The command '/bin/sh -c $VAR' returned a non-zero code: 127
NOTE: it says val1: not found
Mac
Output of docker version:
Client: Docker Engine - Community
Cloud integration: 1.0.4
Version: 20.10.0
API version: 1.41
Go version: go1.13.15
Git commit: 7287ab3
Built: Tue Dec 8 18:55:43 2020
OS/Arch: darwin/amd64
Context: default
Experimental: true
Server: Docker Engine - Community
Engine:
Version: 20.10.0
API version: 1.41 (minimum version 1.12)
Go version: go1.13.15
Git commit: eeddea2
Built: Tue Dec 8 18:58:04 2020
OS/Arch: linux/amd64
Experimental: false
containerd:
Version: v1.4.3
GitCommit: 269548fa27e0089a8b8278fc4fc781d7f65a939b
runc:
Version: 1.0.0-rc92
GitCommit: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
docker-init:
Version: 0.19.0
GitCommit: de40ad0
Output of docker info:
Client:
Context: default
Debug Mode: false
Plugins:
app: Docker App (Docker Inc., v0.9.1-beta3)
buildx: Build with BuildKit (Docker Inc., v0.4.2-docker)
scan: Docker Scan (Docker Inc., v0.5.0)
Server:
Containers: 4
Running: 4
Paused: 0
Stopped: 0
Images: 56
Server Version: 20.10.0
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Native Overlay Diff: true
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.runtime.v1.linux runc io.containerd.runc.v2
Default Runtime: runc
Init Binary: docker-init
containerd version: 269548fa27e0089a8b8278fc4fc781d7f65a939b
runc version: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
init version: de40ad0
Security Options:
seccomp
Profile: default
Kernel Version: 4.19.121-linuxkit
Operating System: Docker Desktop
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 5.809GiB
Name: docker-desktop
ID: JND3:6LEO:X5NW:QRCE:TQKP:P62X:K6FZ:7V2O:3SW6:ON5E:ZLMP:PNSN
Docker Root Dir: /var/lib/docker
Debug Mode: false
HTTP Proxy: gateway.docker.internal:3128
HTTPS Proxy: gateway.docker.internal:3129
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false
Product License: Community Engine
Output from steps
$ docker build -t base_image . -f base.Dockerfile
[+] Building 3.4s (5/5) FINISHED
=> [internal] load build definition from base.Dockerfile 0.1s
=> => transferring dockerfile: 78B 0.0s
=> [internal] load .dockerignore 0.1s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/python:3.7.9 3.1s
=> CACHED [1/1] FROM docker.io/library/python:3.7.9@sha256:91fc2946b9275bb838a468a71e26a96eddd6c31bcf8178cf52108b8cac17240e 0.0s
=> => resolve docker.io/library/python:3.7.9@sha256:91fc2946b9275bb838a468a71e26a96eddd6c31bcf8178cf52108b8cac17240e 0.0s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:bc676b1fca1432f5b26bab380ea7b8372411b9aae48bafbff32c4a0f83e117f7 0.0s
=> => naming to docker.io/library/base_image
$ docker build -t arg_image --build-arg VAR=val2 . -f arg.Dockerfile
[+] Building 0.4s (5/5) FINISHED
=> [internal] load build definition from arg.Dockerfile 0.0s
=> => transferring dockerfile: 40B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/base_image:latest 0.0s
=> CACHED [1/2] FROM docker.io/library/base_image 0.0s
=> ERROR [2/2] RUN val2 0.3s
------
> [2/2] RUN val2:
#5 0.277 /bin/sh: 1: val2: not found
------
executor failed running [/bin/sh -c $VAR]: exit code: 127
NOTE: it says val2: not found
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the two Dockerfiles and build commands from the issue with the Docker Engine versions and builder modes shown for Linux and Mac. Compare how ARG and inherited ENV are resolved, then verify that the chosen precedence is consistent across the reported environments and that the reproduction no longer differs.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- docker, go
- Domain
- build-system, cli
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100