Bake supports loading build definitions from files, but sometimes you need even more flexibility to configure these definitions. For example, you might want to override an attribute when building in a particular environment or for a specific target.

The following list of attributes can be overridden:

  • args
  • cache-from
  • cache-to
  • context
  • dockerfile
  • labels
  • no-cache
  • output
  • platform
  • pull
  • secrets
  • ssh
  • tags
  • target

To override these attributes, you can use the following methods:

File overrides#

You can load multiple Bake files that define build configurations for your targets. This is useful when you want to separate configurations into different files for better organization, or to conditionally override configurations based on which files are loaded.

Default file lookup#

You can use the --file or -f flag to specify which files to load. If you don't specify any files, Bake will use the following lookup order:

  1. compose.yaml
  2. compose.yml
  3. docker-compose.yml
  4. docker-compose.yaml
  5. docker-bake.json
  6. docker-bake.override.json
  7. docker-bake.hcl
  8. docker-bake.override.hcl

If more than one Bake file is found, all files are loaded and merged into a single definition. Files are merged according to the lookup order.

$ docker buildx bake bake --print
[+] Building 0.0s (1/1) FINISHED
 => [internal] load local bake definitions                                                                                                                                                                             0.0s
 => => reading compose.yaml 45B / 45B                                                                                                                                                                                  0.0s
 => => reading docker-bake.hcl 113B / 113B                                                                                                                                                                             0.0s
 => => reading docker-bake.override.hcl 65B / 65B

If merged files contain duplicate attribute definitions, those definitions are either merged or overridden by the last occurrence, depending on the attribute.

Bake will attempt to load all of the files in the order they are found. If multiple files define the same target, attributes are either merged or overridden. In the case of overrides, the last one loaded takes precedence.

For example, given the following files:

```hcl {title=docker-bake.hcl} variable "TAG" { default = "foo" }

target "default" { tags = ["username/my-app:${TAG}"] }


```hcl {title=docker-bake.override.hcl}
variable "TAG" {
  default = "bar"
}

Since docker-bake.override.hcl is loaded last in the default lookup order, the TAG variable is overridden with the value bar.

$ docker buildx bake --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": ["username/my-app:bar"]
    }
  }
}

Manual file overrides#

You can use the --file flag to explicitly specify which files to load, and use this as a way to conditionally apply override files.

For example, you can create a file that defines a set of configurations for a specific environment, and load it only when building for that environment. The following example shows how to load an override.hcl file that sets the TAG variable to bar. The TAG variable is then used in the default target.

```hcl {title=docker-bake.hcl} variable "TAG" { default = "foo" }

target "default" { tags = ["username/my-app:${TAG}"] }


```hcl {title=overrides.hcl}
variable "TAG" {
  default = "bar"
}

Printing the build configuration without the --file flag shows the TAG variable is set to the default value foo.

$ docker buildx bake --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "username/my-app:foo"
      ]
    }
  }
}

Using the --file flag to load the overrides.hcl file overrides the TAG variable with the value bar.

$ docker buildx bake -f docker-bake.hcl -f overrides.hcl --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": [
        "username/my-app:bar"
      ]
    }
  }
}

Command line#

You can also override target configurations from the command line with the --set flag:

# docker-bake.hcl
target "app" {
  args = {
    mybuildarg = "foo"
  }
}

$ docker buildx bake --set app.args.mybuildarg=bar --set app.platform=linux/arm64 app --print

{
  "group": {
    "default": {
      "targets": ["app"]
    }
  },
  "target": {
    "app": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "mybuildarg": "bar"
      },
      "platforms": ["linux/arm64"]
    }
  }
}

Pattern matching syntax defined in https://golang.org/pkg/path/#Match is also supported:

$ docker buildx bake --set foo*.args.mybuildarg=value  # overrides build arg for all targets starting with "foo"
$ docker buildx bake --set *.platform=linux/arm64      # overrides platform for all targets
$ docker buildx bake --set foo*.no-cache               # bypass caching only for targets starting with "foo"

Complete list of attributes that can be overridden with --set are:

  • args
  • cache-from
  • cache-to
  • context
  • dockerfile
  • labels
  • no-cache
  • output
  • platform
  • pull
  • secrets
  • ssh
  • tags
  • target

Environment variables#

You can also use environment variables to override configurations.

Bake lets you use environment variables to override the value of a variable block. Only variable blocks can be overridden with environment variables. This means you need to define the variables in the bake file and then set the environment variable with the same name to override it.

The following example shows how you can define a TAG variable with a default value in the Bake file, and override it with an environment variable.

variable "TAG" {
  default = "latest"
}

target "default" {
  context = "."
  dockerfile = "Dockerfile"
  tags = ["docker.io/username/webapp:${TAG}"]
}

$ export TAG=$(git rev-parse --short HEAD)
$ docker buildx bake --print webapp

The TAG variable is overridden with the value of the environment variable, which is the short commit hash generated by git rev-parse --short HEAD.

{
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": ["docker.io/username/webapp:985e9e9"]
    }
  }
}

Type coercion#

Overriding non-string variables with environment variables is supported. Values passed as environment variables are coerced into suitable types first.

The following example defines a PORT variable with a default value of 8080. The default target uses a ternary operator to set the PORT variable to the value of the environment variable PORT if it is greater than 1024, otherwise it uses the default value.

In this case, the PORT variable is coerced to an integer before the ternary operator is evaluated.

default_port = 8080

variable "PORT" {
  default = default_port
}

target "default" {
  args = {
    PORT = PORT > 1024 ? PORT : default_port
  }
}

Attempting to set the PORT variable with a value less than 1024 will result in the default value being used.

$ PORT=80 docker buildx bake --print

{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "PORT": "8080"
      }
    }
  }
}