Topics

Specification

Dev Container Templates reference

Development Container Templates are source files packaged together that encode configuration for a complete development environment. A Template can be used in a new or existing project, and a supporting tool will use the configuration from the Template to build a development container.

The configuration is placed in a .devcontainer.json which can also reference other files within the Template. Alternatively, .devcontainer/devcontainer.json can also be used if the container needs to reference other files, such as a Dockerfile or docker-compose.yml. A Template can also provide additional source files (eg: boilerplate code or a lifecycle script).

Template metadata is captured by a devcontainer-template.json file in the root folder of the Template.

Folder Structure

A single Template is a folder with at least a devcontainer-template.json and devcontainer.json. Additional files are permitted and are packaged along side the required files.

+-- template
|    +-- devcontainer-template.json
|    +-- .devcontainer.json
|    +-- (other files)

devcontainer-template.json properties

The devcontainer-template.json file defines information about the Template to be used by any supporting tools.

The properties of the file are as follows:

Property	Type	Description
`id`	string	ID of the Template. The `id` should be unique in the context of the repository/published package where the Template exists and must match the name of the directory where the `devcontainer-template.json` resides.
`version`	string	The semantic version of the Template.
`name`	string	Name of the Template.
`description`	string	Description of the Template.
`documentationURL`	string	Url that points to the documentation of the Template.
`licenseURL`	string	Url that points to the license of the Template.
`options`	object	A map of options that the supporting tools should use to populate different configuration options for the Template.
`platforms`	array	Languages and platforms supported by the Template.
`publisher`	string	Name of the publisher/maintainer of the Template.
`keywords`	array	List of strings relevant to a user that would search for this Template.

The `options` property

The options property contains a map of option IDs and their related configuration settings. These options are used by the supporting tools to prompt the user to choose from different Template configuration options. The tools would replace the option ID with the selected value in all the files (within the sub-directory of the Template). This replacement would happen before dropping the .devcontainer.json (or .devcontainer/devcontainer.json) and other files (within the sub-directory of the Template) required to containerize your project. See option resolution for more details. For example:

{
  "options": {
    "optionId": {
      "type": "string",
      "description": "Description of the option",
      "proposals": ["value1", "value2"],
      "default": "value1"
    }
  }
}

Property	Type	Description
`optionId`	string	ID of the option used by the supporting tools to replace the selected value in the files within the sub-directory of the Template.
`optionId.type`	string	Type of the option. Valid types are currently: `boolean`, `string`
`optionId.description`	string	Description for the option.
`optionId.proposals`	array	A list of suggested string values. Free-form values are allowed. Omit when using `optionId.enum`.
`optionId.enum`	array	A strict list of allowed string values. Free-form values are not allowed. Omit when using `optionId.proposals`.
`optionId.default`	string	Default value for the option.

Note: The options must be unique for every devcontainer-template.json

Referencing a Template

The id format (<oci-registry>/<namespace>/<template>[:<semantic-version>]) dictates how a supporting tool will locate and download a given Template from an OCI registry. For example:

ghcr.io/user/repo/go
ghcr.io/user/repo/go:1
ghcr.io/user/repo/go:latest

The registry must implement the OCI Artifact Distribution Specification. Some implementors can be found here.

Versioning

Each Template is individually versioned according to the semver specification. The version property in the respective devcontainer-template.json file is updated to increment the Template’s version.

Tooling that handles releasing Templates will not republish Templates if that exact version has already been published; however, tooling must republish major and minor versions in accordance with the semver specification.

Release

For information on distributing Templates, see the Templates distribution doc.

Option Resolution

A Template’s options property is used by a supporting tool to prompt for different configuration options. A supporting tool will parse the options object provided by the user. If a value is selected for a Template, it will be replaced in the files (within the sub-directory of the Template).

Option resolution example

Consider a java Template with the following folder structure:

+-- java
|    +-- devcontainer-template.json
|    +-- .devcontainer.json

Suppose the java Template has the following options parameters declared in the devcontainer-template.json file:

// ...
"options": {
    "imageVariant": {
        "type": "string",
        "description": "Specify version of java.",
        "proposals": [
          "17-bullseye",
          "17-buster",
          "11-bullseye",
          "11-buster",
          "17",
          "11"
        ],
    "default": "17-bullseye"
    },
    "nodeVersion": {
        "type": "string", 
        "proposals": [
          "latest",
          "16",
          "14",
          "10",
          "none"
        ],
        "default": "latest",
        "description": "Specify version of node, or 'none' to skip node installation."
    },
    "installMaven": {
        "type": "boolean", 
        "description": "Install Maven, a management tool for Java.",
        "default": "false"
    },
}

and it has the following .devcontainer.json file:

{
     "name": "Java",
     "image": "mcr.microsoft.com/devcontainers/java:0-${templateOption:imageVariant}",
     "features": {
          "ghcr.io/devcontainers/features/node:1": {
               "version": "${templateOption:nodeVersion}",
               "installMaven": "${templateOption:installMaven}"
          }
     },
//	...
}

A user tries to add the java Template to their project using the supporting tools and selects 17-bullseye when prompted for "Specify version of Go" and the default values when prompted for "Specify version of node, or 'none' to skip node installation" and "Install Maven, a management tool for Java".

The supporting tool could then use a string replacer for all the files within the sub-directory of the Template. In this example, .devcontainer.json needs to be modified and hence, the inputs can provided to it as follows:

{
  imageVariant:"17-bullseye",
  nodeVersion: "latest",
  installMaven: "false"
}

The modified .devcontainer.json will be as follows:

{
     "name": "Go",
     "image": "mcr.microsoft.com/devcontainers/go:0-17-bullseye",
     "features": {
          "ghcr.io/devcontainers/features/node:1": {
               "version": "latest",
               "installMaven": "false"
	  }
     },
     ...
}

The modified .devcontainer.json would be dropped into any existing folder as a starting point for containerizing your project.