Document language version choice (#304)

Document how to select a non-default language version.

README.md

@@ -32,7 +32,9 @@ Images are pushed to:
 * [Amazon ECR Public Gallery](https://gallery.ecr.aws/pulumi/)
 * [GitHub Container Registry](https://github.com/orgs/pulumi/packages)
 
-## Default Language Versions
+## Language Versions
+
+Images without a version suffix use the following language versions by default:
 
 - .NET 6.0
 - Go 1.21
@@ -57,6 +59,45 @@ Images are pushed to:
 
 Language runtimes are kept up-to-date with current LTS versions. You can pin the image tag to a particular version in order to avoid unintended upgrades.
 
+### Choosing a Language Version
+
+For the language specific slim images, you can choose a specific version of the language runtime by using the suffixed images. For example to use Node.js 22, you would use `pulumi/pulumi-nodejs-22`, for Python 3.12, you would use `pulumi/pulumi-python-3.12`, etc.
+
+For the kitchen sink image (`pulumi/pulumi`), choosing a specific version depends on the language.
+
+#### .NET
+
+The `pulumi/pulumi` image includes .NET 6.0 and 8.0. The `TargetFramework` property in your project’s `.csproj` or `.fsproj` file determines which SDK is used.
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    ...
+```
+
+#### Go
+
+The Go version used to compile your program is determined by the `go` statement in your Pulumi project's `go.mod` file, see [Go Toolchains](https://go.dev/doc/toolchain).
+
+#### Java
+
+The images currently only ship a single version of the JDK, which is 17.
+
+#### Node.js
+
+The `pulumi/pulumi` image uses [fnm](https://github.com/Schniz/fnm) to manage Node.js versions, and comes with the latest patch releases of Node.js 18, 20 and 22 preinstalled. To select a specific version, create a `.node-version` file in your project directory with the desired version number, and run the command `pulumi install --use-language-version-tools`. This will setup the aliases for the image's default version of Node.js, npm, and other Node.js specific tools in `/usr/local/share/fnm/aliases/default/bin`.
+
+To avoid downloading Node.js versions on each run, it is recommended to only specify the major version number, for example `22`. This ensures that the pre-installed version is used.
+
+If you are building your own image by extending the `pulumi/pulumi` image, you can use the `fnm` command `fnm alias ${MY_NODEJS_VERSION} default` to configure the default version of Node.js to be used in your image.
+
+#### Python
+
+The `pulumi/pulumi` image uses [pyenv](https://github.com/pyenv/pyenv) to manage Python versions, and comes with Python 3.9 to 3.12 preinstalled. To select a specific version, create a `.python-version` file in your project directory with the desired version number.
+
+To avoid downloading and building Python versions on each run, it is recommended to only specify the major version number, for example `3.12`. This ensures that the pre-installed version of Python 3.12 is used.
+
 ## Scanning
 
 Images are scanned nightly for vulnerabilities.  Results are checked periodically for issues that can be remediated (best effort), however there are some issues over which we have no control, e.g. vulnerabilities in base images for which there is no known remediation.