Keep readonly-volume-mounted config.json

[toshikish]

Fixes #104.

Bug description

The config.json deletion code

envbuilder/envbuilder.go

Line 775 in 8962d96

err = os.Remove(filepath.Join(os.Getenv("DOCKER_CONFIG"), "config.json"))

seems intended to delete previously generated temporary one.

envbuilder/envbuilder.go

Line 314 in 8962d96

err = os.WriteFile(filepath.Join(MagicDir, "config.json"), decoded, 0644)

But in case config.json file is volume mounted with read-only mode on Docker or Kubernetes, the deletion fails with error: remove docker config: remove /.envbuilder/config.json: device or resource busy.

Bug fix

Delete config.json only if it is generated from DOCKER_CONFIG_BASE64 environment variable.

Validation

Execute the following commands, and a deno devcontainer will run without errors.

echo '{}' > config.json
docker run --rm -it -e GIT_URL=https://github.com/denoland/deno -e INIT_SCRIPT=bash -v ./config.json:/.envbuilder/config.json:ro envbuilder:latest