ls: improve handling of invalid builders/context, clarify relation with context

[thaJeztah]

Related (see further down):

• Difficult to execute commands when currentContext is invalid cli#1955
• context: implement lazy loading, and other improvements cli#3847
• context: don't validate context when looking up name cli#3878
• cli/command/context: "docker context show": don't validate context cli#3886
• docker context rm: allow --force to ignore non-existing contexts cli#3791

⚠️ Disclaimer: I just ran into this issue on buildx v0.9.1 (Docker Desktop 4.16.1), but have not yet verified the behavior on v0.10.0;

The issue: an invalid builder (context)

When running docker buildx ls, I noticed an error message:

docker buildx ls
NAME/NODE       DRIVER/ENDPOINT STATUS  BUILDKIT                         PLATFORMS
default *       docker
  default       default         running 22.06.0-beta.0-913-g6be7129598.m linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/mips64le, linux/mips64
desktop-linux   docker
  desktop-linux desktop-linux   running 22.06.0-beta.0-913-g6be7129598.m linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/mips64le, linux/mips64
foo                             error

Cannot load builder foo: Unix socket path "/var/folders/6f/tz5jf4nn1_n5jb0ctrrw5p2w0000gn/T/TestBuildWithBuildercustom_context67217891/001/docker.sock" is too long

Apparently I have created a custom builder for testing, and that builder could not be used (path too long), so my (logical) approach was to "just remove the builder";

docker buildx rm foo
ERROR: Unix socket path "/var/folders/6f/tz5jf4nn1_n5jb0ctrrw5p2w0000gn/T/TestBuildWithBuildercustom_context67217891/001/docker.sock" is too long

That failed, and the builder is still there;

docker buildx ls
NAME/NODE       DRIVER/ENDPOINT STATUS  BUILDKIT                         PLATFORMS
default *       docker
  default       default         running 22.06.0-beta.0-913-g6be7129598.m linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/mips64le, linux/mips64
desktop-linux   docker
  desktop-linux desktop-linux   running 22.06.0-beta.0-913-g6be7129598.m linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/mips64le, linux/mips64
foo                             error

Cannot load builder foo: Unix socket path "/var/folders/6f/tz5jf4nn1_n5jb0ctrrw5p2w0000gn/T/TestBuildWithBuildercustom_context67217891/001/docker.sock" is too long

Relation with "context"

So, I guess the issue here is because docker buildx ls doesn't actually list builders, but lists contexts (and checks each context to see if there's a builder?). This is confusing.

There is indeed a context named foo, and that has a Docker Endpoint with a socket path that's too long (this may be something we should validate when creating the context);

docker context ls
NAME                TYPE                DESCRIPTION                               DOCKER ENDPOINT                                                                                                      KUBERNETES ENDPOINT   ORCHESTRATOR
default *           moby                Current DOCKER_HOST based configuration   unix:///var/run/docker.sock                                                                                                                swarm
desktop-linux       moby                                                          unix:///Users/thajeztah/.docker/run/docker.sock
foo                 moby                                                          unix:///var/folders/6f/tz5jf4nn1_n5jb0ctrrw5p2w0000gn/T/TestBuildWithBuildercustom_context67217891/001/docker.sock

Removing the context worked:

docker context rm foo
foo

And resolved the issue:

docker buildx ls
NAME/NODE       DRIVER/ENDPOINT STATUS  BUILDKIT                         PLATFORMS
default *       docker
  default       default         running 22.06.0-beta.0-913-g6be7129598.m linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/mips64le, linux/mips64
desktop-linux   docker

[thaJeztah]

/cc @crazy-max @dvdksn