build: add variable validation in bake

[dvdksn]

Signed-off-by: David Karlsson [email protected]

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[crazy-max]

Nice thanks! These are great examples 🎉

[crazy-max]

Wonder if we could show another example where a variable depends on another one like: docker/buildx#2794 (comment)

variable "FOO" {}
variable "BAR" {
  validation {
    condition = FOO != ""
    error_message = "BAR requires FOO to be set."
  }
}

target "default" {
  args = {
    BAR = BAR
  }
}

$ docker buildx bake --print
#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 183B / 183B done
#1 DONE 0.0s
docker-bake.hcl:4
--------------------
   2 |     variable "BAR" {
   3 |       validation {
   4 | >>>     condition = FOO != ""
   5 |         error_message = "BAR requires FOO to be set."
   6 |       }
--------------------
ERROR: docker-bake.hcl:4,17-26: Validation failed; BAR requires FOO to be set.

[dvdksn]

@crazy-max updated the examples a little (added a dependency one, and clarified the existing ones)

[crazy-max]

❤️