make bake definitions extensible for downstream overrides

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

Author: crazy-max

README.md

@@ -17,6 +17,7 @@ ___
 * [Build](#build)
   * [Requirements](#requirements)
   * [Usage](#usage)
+  * [Remote override](#remote-override)
 * [Contributing](#contributing)
 
 ## Release
@@ -81,6 +82,54 @@ $ PKG_REF=v0.24.0 docker buildx bake pkg-buildx-*
 $ docker buildx bake --push --set *.tags=dockereng/packaging:buildx-v0.24.0 release-buildx 
 ```
 
+### Remote override
+
+Downstream repositories can reuse this repository as a remote Bake definition
+and extend it with a local `docker-bake.override.hcl` file instead of forking
+the full `docker-bake.hcl`.
+
+Example local override:
+
+```hcl
+variable "PKGS_EXTRA" {
+  default = ["my-package"]
+}
+
+variable "PKG_PLATFORMS_EXTRA" {
+  default = {
+    my-package = ["linux/amd64"]
+  }
+}
+
+variable "PKG_CONTEXTS_EXTRA" {
+  default = {
+    my-package = "cwd://pkg/my-package"
+  }
+}
+
+target "_pkg-my-package" {
+  args = {
+    PKG_NAME = "my-package"
+    PKG_REPO = "https://github.com/example/my-package.git"
+    PKG_REF = "main"
+  }
+}
+```
+
+Example invocation from the downstream repository:
+
+```shell
+$ docker buildx bake \
+  -f docker-bake.hcl \
+  -f cwd://docker-bake.override.hcl \
+  "https://github.com/docker/packaging.git#main" \
+  pkg-my-package-debian12
+```
+
+Use the `cwd://` prefix for local files and directories when combining a local
+override with a remote Bake definition. Without it, Bake resolves paths relative
+to the remote context instead of the current working directory.
+
 ## Contributing
 
 Want to contribute? Awesome! You can find information about contributing to

docker-bake.hcl

@@ -42,8 +42,8 @@ variable "DISTROS" {
   ]
 }
 
-variable "PKGS" {
-  description = "List of packages to build from ./pkg directory. Don't forget to update _pkg-* target if you add/remove a package."
+variable "PKGS_BASE" {
+  description = "Base list of packages built from this repo. Use PKGS_EXTRA in an override file to add downstream packages."
   default = [
     "buildx",
     "compose",
@@ -56,6 +56,49 @@ variable "PKGS" {
   ]
 }
 
+variable "PKGS_EXTRA" {
+  description = "Additional packages to append from an override bake definition."
+  default = []
+}
+
+variable "PKGS" {
+  description = "Full list of packages to build from ./pkg directory or an override-provided package context."
+  default = concat(PKGS_BASE, PKGS_EXTRA)
+}
+
+variable "PKG_PLATFORMS_BASE" {
+  description = "Base package platform mapping. Use PKG_PLATFORMS_EXTRA in an override bake definition to add downstream packages."
+  default = {
+    # https://github.com/docker/buildx/blob/0c747263ef1426f5fa217fcdb616eddf33da6c2d/docker-bake.hcl#L156-L174
+    buildx = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x", "windows/amd64", "windows/arm64"]
+    # https://github.com/docker/compose/blob/c626befee1596abcc74578cb10dd96ae1667f76f/docker-bake.hcl#L112-L124
+    compose = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x", "windows/amd64", "windows/arm64"]
+    # https://github.com/containerd/containerd/blob/e373060a953aeaa35554e2f667043fed73ff6248/.github/workflows/ci.yml#L142-L162
+    # https://github.com/containerd/containerd/blob/e373060a953aeaa35554e2f667043fed73ff6248/.github/workflows/ci.yml#L133-L134
+    containerd = ["linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/s390x", "windows/amd64", "windows/arm64"]
+    # https://github.com/docker/docker-credential-helpers/blob/f9d3010165b642df37215b1be945552f2c6f0e3b/docker-bake.hcl#L56-L66
+    credential-helpers = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/s390x", "windows/amd64"]
+    # https://github.com/docker/cli/blob/84038691220e7ba3329a177e4e3357b4ee0e3a52/docker-bake.hcl#L30-L42
+    docker-cli = ["darwin/amd64", "darwin/arm64", "linux/386", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x", "windows/amd64", "windows/arm64"]
+    # https://github.com/moby/moby/blob/83264918d3e1c61341511e360a7277150b914b3f/docker-bake.hcl#L82-L91
+    docker-engine = ["linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/s390x", "windows/amd64", "windows/arm64"]
+    # https://github.com/docker/model-runner/blob/039f7a31c0365f9161c9b9b6bb3888161d16e388/cmd/cli/Makefile#L39-L43
+    model = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm64", "linux/arm/v7", "windows/amd64", "windows/arm64"]
+    # https://github.com/docker/docker-agent/blob/5b9feaabe743a5ad577f2247ed55d5dcb2678e8b/Taskfile.yml#L79
+    agent = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm64", "windows/amd64", "windows/arm64"]
+  }
+}
+
+variable "PKG_PLATFORMS_EXTRA" {
+  description = "Additional package platform mapping to merge from an override bake definition."
+  default = {}
+}
+
+variable "PKG_CONTEXTS_EXTRA" {
+  description = "Additional package context mapping to merge from an override bake definition."
+  default = {}
+}
+
 variable "DISTRO_NAME" {
   description = "Name of the distro."
   default = null
@@ -589,25 +632,12 @@ target "_pkg-agent" {
 # Returns the list of supported platforms for a given package.
 function "pkgPlatforms" {
   params = [pkg]
-  result = lookup({
-    # https://github.com/docker/buildx/blob/0c747263ef1426f5fa217fcdb616eddf33da6c2d/docker-bake.hcl#L156-L174
-    buildx = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x", "windows/amd64", "windows/arm64"]
-    # https://github.com/docker/compose/blob/c626befee1596abcc74578cb10dd96ae1667f76f/docker-bake.hcl#L112-L124
-    compose = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x", "windows/amd64", "windows/arm64"]
-    # https://github.com/containerd/containerd/blob/e373060a953aeaa35554e2f667043fed73ff6248/.github/workflows/ci.yml#L142-L162
-    # https://github.com/containerd/containerd/blob/e373060a953aeaa35554e2f667043fed73ff6248/.github/workflows/ci.yml#L133-L134
-    containerd = ["linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/s390x", "windows/amd64", "windows/arm64"]
-    # https://github.com/docker/docker-credential-helpers/blob/f9d3010165b642df37215b1be945552f2c6f0e3b/docker-bake.hcl#L56-L66
-    credential-helpers = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/s390x", "windows/amd64"]
-    # https://github.com/docker/cli/blob/84038691220e7ba3329a177e4e3357b4ee0e3a52/docker-bake.hcl#L30-L42
-    docker-cli = ["darwin/amd64", "darwin/arm64", "linux/386", "linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x", "windows/amd64", "windows/arm64"]
-    # https://github.com/moby/moby/blob/83264918d3e1c61341511e360a7277150b914b3f/docker-bake.hcl#L82-L91
-    docker-engine = ["linux/amd64", "linux/arm/v6", "linux/arm/v7", "linux/arm64", "linux/ppc64le", "linux/s390x", "windows/amd64", "windows/arm64"]
-    # https://github.com/docker/model-runner/blob/039f7a31c0365f9161c9b9b6bb3888161d16e388/cmd/cli/Makefile#L39-L43
-    model = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm64", "linux/arm/v7", "windows/amd64", "windows/arm64"]
-    # https://github.com/docker/docker-agent/blob/5b9feaabe743a5ad577f2247ed55d5dcb2678e8b/Taskfile.yml#L79
-    agent = ["darwin/amd64", "darwin/arm64", "linux/amd64", "linux/arm64", "windows/amd64", "windows/arm64"]
-  }, pkg, [])
+  result = lookup(merge(PKG_PLATFORMS_BASE, PKG_PLATFORMS_EXTRA), pkg, [])
+}
+
+function "pkgContext" {
+  params = [pkg]
+  result = lookup(PKG_CONTEXTS_EXTRA, pkg, "./pkg/${pkg}")
 }
 
 #
@@ -652,7 +682,7 @@ target "pkg" {
     pkg = PKGS
     distro = DISTROS
   }
-  context = "./pkg/${pkg}"
+  context = pkgContext(pkg)
   contexts = {
     scripts = "./hack/scripts"
   }
@@ -675,7 +705,7 @@ target "verify" {
     pkg = PKGS
     distro = DISTROS
   }
-  context = "./pkg/${pkg}"
+  context = pkgContext(pkg)
   dockerfile = "verify.Dockerfile"
   contexts = {
     scripts = "./hack/scripts"
@@ -708,7 +738,7 @@ target "metadata" {
   matrix = {
     pkg = PKGS
   }
-  context = "./pkg/${pkg}"
+  context = pkgContext(pkg)
   contexts = {
     scripts = "./hack/scripts"
   }