[0.13+] Creating a `docker context` automatically creates an unwanted `docker buildx` instance

[Victor Ros]

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• I've read the contributing guidelines and wholeheartedly agree

I've found a bug and checked that ...

• ... the documentation does not mention anything about my problem
• ... there are no open or closed issues that are related to my problem

Description

Hello everyone!

I didn't find any issue about docker context or docker buildx creation issue since 0.13 similar to what I got.

Since 0.13, I cannot create a multi-arch buildx instance anymore if I create a docker context also named multi-arch.
It seems that it automatically creates an "unwanted" buildx instance named multi-arch when docker context multi-arch is created.

I didn't have this behavior in 0.12 and didn't notice a clear change of that in 0.13 release note.

I shared my local information as I also encountered this problem when I recently update Rancher Desktop on my local machine because Rancher Desktop upgrades buildx to 0.13.0.
I first detected this problem in our pipeline agents.
We create an image where we install Docker and create a multi-arch context and buildx instance to be able to build AMD64 and ARM64 images.
Since this problem occurred, our team cannot update the image used by our pipeline agents.

Error occurred when docker context is created and the command to create the docker buildx instance is run.
It tell us that the instance already exists (cf. Actual behaviour)

ERROR: instance name "multi-arch" already exists as context builder

Expected behaviour

docker context create \
  --description "Multi-architecture context (AMD64/ARM64)" \
  --docker "host=unix:///run/user/$(id -u)/docker.sock" \
  multi-arch

# Successful output
multi-arch
Successfully created context "multi-arch"

docker buildx create \
  --bootstrap \
  --driver docker-container \
  --name multi-arch \
  --node multi-arch \
  --platform linux/amd64,linux/arm64 \
  --use \
  multi-arch

# Successful output
multi-arch

Actual behaviour

docker context create \
  --description "Multi-architecture context (AMD64/ARM64)" \
  --docker "host=unix:///run/user/$(id -u)/docker.sock" \
  multi-arch

# Successful output
multi-arch
Successfully created context "multi-arch"

docker buildx create \
  --bootstrap \
  --driver docker-container \
  --name multi-arch \
  --node multi-arch \
  --platform linux/amd64,linux/arm64 \
  --use \
  multi-arch

# Unsuccessful output
ERROR: instance name "multi-arch" already exists as context builder

Buildx version

github.com/docker/buildx v0.13.0 0de5f1c

Docker info

Client:
 Version:    25.0.4-rd
 Context:    rancher-desktop
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.13.0
    Path:     /Users/ros/.docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.24.7
    Path:     /Users/ros/.docker/cli-plugins/docker-compose

Server:
 Containers: 1
  Running: 1
  Paused: 0
  Stopped: 0
 Images: 47
 Server Version: 24.0.7
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 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 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 4e1fe7492b9df85914c389d1f15a3ceedbb280ac
 runc version: 18a0cb0f32bcac2ecc9a10f327d282759c144dab
 init version:
 Security Options:
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.6.14-0-virt
 Operating System: Alpine Linux v3.19
 OSType: linux
 Architecture: aarch64
 CPUs: 4
 Total Memory: 5.787GiB
 Name: lima-rancher-desktop
 ID: 90310736-7374-4a85-ba14-1699fce374d8
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Builders list

NAME/NODE             DRIVER/ENDPOINT       STATUS    BUILDKIT               PLATFORMS
multi-arch            docker
 \_ multi-arch         \_ multi-arch        running   v0.11.7+d3e6c1360f6e   linux/arm64
rancher-desktop*      docker
 \_ rancher-desktop    \_ rancher-desktop   running   v0.11.7+d3e6c1360f6e   linux/arm64
default                                     error

# Important Note: This output is produced just after I run "docker context create" command

Configuration

I'm not even at the Dockerfile stage yet 😂

Build logs

No response

Additional info

No response

[Tõnis Tiigi]

All docker contexts always come with a builder instance to use the buildkit that is embedded in Docker daemon. This has been like this from the beginning of buildx creation. When you run docker build that redirects to docker buildx build of the active Docker context's builder.

What changed in v0.13 is that we now have stricter validation for creating builders when one with same name already exists or appending nodes to builders when no name existed. This is to avoid errors with one builder accidentally shadowing another, leading to unexpected behavior.

[Victor Ros]

Thank you for the explanation.

What we were doing was not correct.
From what I understood, we only need to create the buildx instance without having to create a dedicated context.

It should be sufficient to configure the multi-arch buildx instance without any errors.

Something like that:

docker buildx create \
  --bootstrap \
  --driver docker-container \
  --name multi-arch \
  --node multi-arch \
  --platform linux/amd64,linux/arm64 \
  --use

[thompson-shaun]

OK to close this @VictorRos?