Buildx target.matrix CLI override

[Nithos]

Hi,
I have a bakefile that leverages the matrix to build common images which work great. What I'm wondering about is if there is a way to trigger (or override in the CLI) to build just one of the targets defined in the matrix. I'm hoping to just be able to build one of them during development/debugging rather than all targets defined. The only way I can think of is breaking them apart which negates the benefits of the matrix.

Example

target "app" {
  name = "app-${tgt}"
  matrix = {
    tgt = ["foo", "bar"]
  }
  target = tgt
}

running docker buildx bake app will build both app-foo and app-bar is there a way to only trigger building foo-bar?
Looking at the documentation it does not appear that using the --set override command will work here (at least not according to the docs).

Any ideas?

[dvdksn]

How about using a ternary/variable to check for an override? Something like:

variable "MATRIX_OVERRIDE" { default="" }

function "list" {
  params = []
  variadic_params = items
  result = items
}

target "app" {
  name = "app-${tgt}"
  matrix = {
    tgt = notequal("",MATRIX_OVERRIDE) ? list(MATRIX_OVERRIDE) : list("foo", "bar")
  }
  target = tgt
}

[Nithos]

This is great, yes that should work at least for my current simple case. Would still be nice if there was another way doing this with the current tools (i.e with --set).

[lemeurherve]

My current workaround is to use the string of a JSON array as environment variable that is then jsondecoded in a function to retrieve the tuple for the matrix.

It works but it's kinda ugly:

variable "MY_CHOICES" {
  default = "[\"one\",\"two\",\"three\"]"
}

function "mychoices" {
  params = [choices]
  result = jsondecode(choices)
}

target "mytarget" {
  matrix = {
    choice = mychoices(MY_CHOICES)
  }
  name       = "mytarget-${choice}"
}

MY_CHOICES='["four","five"]' docker buildx bake

I didn't find a way to directly define the tuple content as env var that the HCL parser could resolve as tuple to allow overriding it:

variable "mychoices" {
  default = ["one", "two", "three"]
}

target "mytarget" {
  matrix = {
    choice = mychoices
  }
  name       = "mytarget-${choice}"
}

Current error:

$ mychoices='["four","five"]' docker buildx bake
 1 | >>> variable "mychoices" {
 2 |       default = ["one", "two", "three"]
 3 |     }
--------------------
ERROR: bake.hcl: Invalid value; unsupported type tuple for variable mychoices

I'd love to be able to set variables of typetuple or list from either env var or either the command line so they could be used in matrix without resorting to this trick which is a bit confusing.

[lemeurherve]

Ref:

• https://docs.docker.com/build/bake/configuring-build/#from-command-line
• https://docs.docker.com/build/bake/advanced/#typed-variables
• The terraform doc explaining how to declare tuple from env var, letting me hope I could do the same with bake (also using HCL): https://developer.hashicorp.com/terraform/language/values/variables#complex-typed-values

[lemeurherve]

I also tried to add a type constraint to the variable like we can do with HCL for terraform but the bake HCL parser doesn't recognise it.

https://developer.hashicorp.com/terraform/language/expressions/types