bake: add semvercmp func to stdlib

[crazy-max]

variable "ALPINE_VERSION" {
  default = "3.23"
}

target "default" {
  dockerfile = "Dockerfile.webapp"
  platforms = semvercmp(ALPINE_VERSION, ">= 3.20") ? [
    "linux/amd64",
    "linux/arm64",
    "linux/riscv64"
  ] : [
    "linux/amd64",
    "linux/arm64"
  ]
}

$ docker buildx bake --print
#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 275B / 275B done
#1 DONE 0.0s
{
  "group": {
    "default": {
      "targets": [
        "default"
      ]
    }
  },
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile.webapp",
      "platforms": [
        "linux/amd64",
        "linux/arm64",
        "linux/riscv64"
      ]
    }
  }
}

$ ALPINE_VERSION=3.19 docker buildx bake --print
#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 275B / 275B done
#1 DONE 0.0s
{
  "group": {
    "default": {
      "targets": [
        "default"
      ]
    }
  },
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile.webapp",
      "platforms": [
        "linux/amd64",
        "linux/arm64"
      ]
    }
  }
}

$ ALPINE_VERSION=latest docker buildx bake --print
#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 275B / 275B done
#1 DONE 0.0s
docker-bake.hcl:7
--------------------
   5 |     target "default" {
   6 |       dockerfile = "Dockerfile.webapp"
   7 | >>>   platforms = semvercmp(ALPINE_VERSION, ">= 3.20") ? [
   8 |         "linux/amd64",
   9 |         "linux/arm64",
--------------------
ERROR: docker-bake.hcl:7,15-32: Error in function call; Call to function "semvercmp" failed: invalid semantic version., and 1 other diagnostic(s)

[dvdksn]

Maybe drop a link to https://github.com/Masterminds/semver here, for the constraint syntax reference.

(I know this doc is pretty bare atm, need to do a pass on this at some point.)

[crazy-max]

Added link to docs

[dvdksn]

LGTM

[tonistiigi]

What about smth like semvercmp, semvercheck?