docs: Add undocumented environment variables to build-container-image manual (#129)

Motivation
----------

Where command line options also have corresponding environment
variables, the variables should be documented. This PR documents a few
variables which had been overlooked.

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

Add descriptions of previously undocumented environment variables:
* `CONTAINERTOOL_ARCHITECTURE`
* `CONTAINERTOOL_DEFAULT_USERNAME`
* `CONTAINERTOOL_DEFAULT_PASSWORD`

Result
------

* All current configuration variables are documented.

Test Plan
---------

* No functional change.
* All existing tests continue to pass.

Author: euanh

Sources/swift-container-plugin/Documentation.docc/build-container-image.md

@@ -91,6 +91,21 @@ Wrap a binary in a container image and publish it.
   Base image on which to layer the application.
   (default: `swift:slim`)
 
+- term `CONTAINERTOOL_ARCHITECTURE`:
+  CPU architecture.
+
 - term `CONTAINERTOOL_OS`:
   Operating system to encode in the container image.
   (default: `Linux`)
+
+- term `CONTAINERTOOL_DEFAULT_USERNAME`:
+  Default username to use when logging into the registry.
+
+  This username is used if there is no matching `.netrc` entry for the registry, there is no `.netrc` file, or the `--disable-netrc` option is set.
+  The same username is used for the source and destination registries.
+
+- term `CONTAINERTOOL_DEFAULT_PASSWORD`:
+  Default password to use when logging into the registry.
+
+  This password is used if there is no matching `.netrc` entry for the registry, there is no `.netrc` file, or the `--disable-netrc` option is set.
+  The same password is used for the source and destination registries.