docs: Add reference documentation for plugin (#109)

Motivation
----------

Reference documentation, such as manual pages, complements getting
started guides and tutorials by explaining a tool's interface and
options in one place.

The `build-container-image` command plugin is the main user-facing tool
in the project, so users will benefit from having reference
documentation for it, available from the main documentation site.

At present, `containertool` will mostly be used through the plugin,
which is a thin wrapper which calls `containertool` as a helper.
Documenting `containertool` as a separate tool might be confusing. It
has command line help for those who need to use it directly, and as the
plugin is largely a wrapper its reference documentation also explains
most of `containertool`'s options.

Modifications
-------------

* Rewrite the `containertool` manual page for the
`build-container-image` plugin.
* Add a Reference section to the curation, linking to the plugin manual
page.

Result
------

Users have access to reference documentation which explains the
interface and options of the `build-container-image` command plugin.

Test Plan
---------

All existing tests continue to pass.

Author: euanh

Sources/containertool/Documentation.docc/containertool.md

@@ -77,3 +77,6 @@ Take a look at some [other examples](https://github.com/apple/swift-container-pl
 ### Building and running
 - <doc:build>
 - <doc:run>
+
+### Reference
+- <doc:build-container-image>

Sources/swift-container-plugin/Documentation.docc/Swift-Container-Plugin.md

@@ -0,0 +1,91 @@
+# build-container-image plugin
+
+Wrap a binary in a container image and publish it.
+
+## Overview
+
+`build-container-image` is a Swift Package Manager [command plugin](https://github.com/swiftlang/swift-package-manager/blob/main/Documentation/Plugins.md#using-a-package-plugin) which packages a product defined in `Package.swift` in a container image and publishes it to repository on a container registry.
+
+### Usage
+
+`swift package build-container-image [<options>] --repository <repository>`
+
+### Options
+
+- term `--product <product>`:
+  The name of the product to package.
+
+  If `Package.swift` defines only one product, it will be selected by default.
+
+- term  `--default-registry <default-registry>`:
+  The default registry hostname. (default: `docker.io`)
+
+  If the value of the `--repository` argument does not contain a registry hostname, the default registry will be prepended to the repository path.
+
+- term  `--repository <repository>`:
+  The repository path.
+
+  If the path does not begin with a registry hostname, the default registry will be prepended to the path.
+
+- term  `--resources <resources>`:
+  Add the file or directory at `resources` to the image.
+  Directories are added recursively.
+
+  If the `product` being packaged has a [resource bundle](https://developer.apple.com/documentation/xcode/bundling-resources-with-a-swift-package) it will be added to the image automatically.
+
+- term  `--username <username>`:
+  Username to use when logging into the registry.
+
+  The same username is used for the source and destination registries.
+  The `.netrc` file is ignored when this option is specified.
+
+- term  `--password <password>`:
+  Password to use when logging into the registry.
+
+  The same password is used for the source and destination registries.
+  The `.netrc` file is ignored when this option is specified.
+
+- term  `-v, --verbose`:
+  Verbose output.
+
+- term  `--allow-insecure-http <allow-insecure-http>`:
+  Connect to the container registry using plaintext HTTP. (values: `source`, `destination`, `both`)
+
+- term  `--architecture <architecture>`:
+  CPU architecture to record in the image.
+
+- term  `--from <from>`:
+  Base image reference. (default: `swift:slim`)
+
+  If the base image is `scratch`, the final image will have no base layer and will consist only of the application layer and resource bundle layer, if the product has a resource bundle.
+
+- term  `--os <os>`:
+  Operating system to record in the image. (default: `linux`)
+
+- term  `--tag <tag>`:
+  Tag for this manifest.
+
+  The `latest` tag is automatically updated to refer to the published image.
+
+- term  `--enable-netrc/--disable-netrc`:
+  Load credentials from a netrc file (default: `--enable-netrc`)
+
+- term  `--netrc-file <netrc-file>`:
+  The path to the `.netrc` file.
+
+- term  `-h, --help`:
+  Show help information.
+
+### Environment
+
+- term `CONTAINERTOOL_DEFAULT_REGISTRY`:
+  Default image registry hostname, used when the `--repository` argument does not contain a registry hostname.
+  (default: `docker.io`)
+
+- term `CONTAINERTOOL_BASE_IMAGE`:
+  Base image on which to layer the application.
+  (default: `swift:slim`)
+
+- term `CONTAINERTOOL_OS`:
+  Operating system to encode in the container image.
+  (default: `Linux`)