Fix default value for build args in docker workflow

[sirewix]

github-workflows/.github/workflows/docker.yml

Lines 44 to 48 in ebd46e1

	build_args:
	description: "List of build arguments, as an array inside a string"
	required: false
	default: "[]"
	type: string

But according to docs:

List type is a newline-delimited string

| build-args | List | List of build-time variables |