Add extends property for configuration inheritance

Document and schema-enable the extends keyword from #22, merging referenced files with the existing image metadata merge logic.

Author: SamuelFrost

docs/specs/devcontainer-reference.md

@@ -84,7 +84,7 @@ To apply the metadata together with a user's `devcontainer.json` at runtime the
 | `updateRemoteUserUID` | `boolean` | Last value wins. | ✓ |   |
 | `hostRequirements` | `cpus`, `memory`, `storage`, `gpu` | Max value wins. | ✓ |   |
 
-Variables in string values will be substituted at the time the value is applied. When the order matters, the `devcontainer.json` is considered last.
+Variables in string values will be substituted at the time the value is applied. When the order matters, the `devcontainer.json` is considered last. The same merge logic is used when a `devcontainer.json` file [extends](devcontainerjson-reference.md#configuration-inheritance) another configuration file in the same repository.
 
 ### Notes
 

docs/specs/devcontainerjson-reference.md

@@ -9,6 +9,7 @@ Metadata properties marked with a 🏷️ can be stored in the `devcontainer.met
 | Property | Type | Description |
 |----------|------|-------------|
 | `name` | string | A name for the dev container displayed in the UI |
+| `extends` | string | A relative path to a JSON or JSONC file in the same repository to use as a base configuration. The referenced file is merged with this file using the [image metadata merge logic](devcontainer-reference.md#merge-logic). See [Configuration inheritance](#configuration-inheritance). |
 | `forwardPorts` 🏷️ | array | An array of port numbers or `"host:port"` values  (e.g. `[3000, "db:5432"]`) that should always be forwarded from inside the primary container to the local machine (including on the web). The property is most useful for forwarding ports that cannot be auto-forwarded because the related process that starts before the `devcontainer.json` supporting service / tool connects or for forwarding a service not in the primary container in Docker Compose scenarios (e.g. `"db:5432"`). Defaults to `[]`. |
 | `portsAttributes` 🏷️ | object | Object that maps a port number, `"host:port"` value, range, or regular expression to a set of default options. See [port attributes](#port-attributes) for available options. For example: <br />`"portsAttributes": {"3000": {"label": "Application port"}}` |
 | `otherPortsAttributes` 🏷️ | object | Default options for ports, port ranges, and hosts that aren't configured using `portsAttributes`. See [port attributes](#port-attributes) for available options. For example: <br /> `"otherPortsAttributes": {"onAutoForward": "silent"}` |
@@ -29,6 +30,44 @@ Metadata properties marked with a 🏷️ can be stored in the `devcontainer.met
 | `overrideFeatureInstallOrder` | array | By default, Features will attempt to automatically set the order they are installed based on a `installsAfter` property within each of them. This property allows you to override the Feature install order when needed. For example: <br />`"overrideFeatureInstallОrder": [ "ghcr.io/devcontainers/features/common-utils", "ghcr.io/devcontainers/features/github-cli" ]` |
 | `customizations` 🏷️| object | Product specific properties, defined in [supporting tools](supporting-tools.md) |
 
+## Configuration inheritance
+
+Multiple teams collaborating on a common codebase may need slightly different `devcontainer.json` settings. The `extends` property lets a configuration inherit from another JSON or JSONC file in the same repository:
+
+```jsonc
+// .devcontainer/defaults.json
+{
+	"name": "example/project",
+	"forwardPorts": [80, 5432],
+	"hostRequirements": {
+		"storage": "64gb",
+		"memory": "16gb"
+	}
+}
+
+// .devcontainer/devcontainer.json
+{
+	"extends": "./defaults.json",
+	"forwardPorts": [2222],
+	"hostRequirements": {
+		"memory": "32gb"
+	},
+	"onCreateCommand": ".devcontainer/on-create-command.sh"
+}
+```
+
+`extends` is a path relative to the file that declares it (for example `"./defaults.json"`, `"../defaults.json"`, or `"./dev/defaults.json"`). Referenced files may themselves use `extends`. Absolute paths and URLs are not supported.
+
+The referenced configuration is merged with the current file using the same [merge logic](devcontainer-reference.md#merge-logic) applied to image metadata, with the current file considered last:
+
+- Array properties such as `forwardPorts`, `capAdd`, and `securityOpt` are the union of values without duplicates.
+- `hostRequirements` takes the maximum of each field.
+- Object maps such as `remoteEnv`, `containerEnv`, `features`, and `customizations` merge per key, with the current file winning on conflicts.
+- Boolean `init` and `privileged` are `true` if at least one value is `true`.
+- Scalar properties such as `name`, `image`, `remoteUser`, and lifecycle commands use last value wins.
+
+The `extends` property itself is not present in the merged result.
+
 ## Scenario specific properties
 
 The focus of `devcontainer.json` is to describe how to enrich a container for the purposes of development rather than acting as a multi-container orchestrator format. Instead, container orchestrator formats can be referenced when needed to manage multiple containers and their lifecycles. Today, `devcontainer.json` includes scenario specific properties for working without a container orchestrator (by directly referencing an image or Dockerfile) and for using Docker Compose as a simple multi-container orchestrator.

schemas/devContainer.base.schema.json

@@ -16,6 +16,10 @@
 					"type": "string",
 					"description": "A name for the dev container which can be displayed to the user."
 				},
+				"extends": {
+					"type": "string",
+					"description": "A relative path to a JSON or JSONC file in the same repository whose configuration is used as a base. The referenced file is merged with this file using the image metadata merge logic."
+				},
 				"features": {
 					"type": "object",
 					"description": "Features to add to the dev container.",