README: add docs for docker builds

Updates tailscale/tailscale#8905

Signed-off-by: Will Norris <will@tailscale.com>

README.md

@@ -29,20 +29,43 @@ In those cases, this plugin may be helpful.
 [automatic certificates]: https://caddyserver.com/docs/automatic-https#activation
 [authenticate Tailscale users]: https://caddyserver.com/docs/caddyfile/directives/forward_auth#tailscale
 
-## Installation
+## Building
 
 Use [xcaddy](https://github.com/caddyserver/xcaddy) to build Caddy with the Tailscale plugin included.
 
 ```sh
 xcaddy build v2.8.4 --with github.com/tailscale/caddy-tailscale
 ```
 
-Alternatively, you can build the included `cmd/caddy` package:
+Alternatively, you can build the included `cmd/caddy` package, or create your own similar package:
 
 ```sh
 go build ./cmd/caddy
 ```
 
+### Docker Builds
+
+Caddy provides [builder docker images] (e.g. `caddy:2.8-builder`) that have xcaddy pre-installed.
+These can be used to build caddy-tailscale in a docker container,
+but may result in issues with the Go version used for the build due to project release cycles.
+
+The caddy builder images are built with the specific Go version that was used for that Caddy version.
+If a new version of Go has been released, that will not be reflected until a future Caddy release.
+Tailscale, and by extension the caddy-tailscale plugin, adopts new Go versions much more quickly,
+which may result in a caddy-tailscale version requiring a more recent version of Go than is in the caddy builder image.
+
+This can be addressed in two ways:
+
+1.  don't use the caddy builder images directly, but install xcaddy in a newer Go build image
+    as [discussed in this comment].
+2.  unset the `GOTOOLCHAIN` environment variable so that Go is able to upgrade itself:
+    ```sh
+    docker run -e GOTOOLCHAIN= -i -t --rm caddy:2.7-builder sh -c "xcaddy build --with github.com/tailscale/caddy-tailscale@0f105e89fbe2222c690b94b5b0b2a8150fa2540f"
+    ```
+
+[builder docker images]: https://hub.docker.com/_/caddy
+[discussed in this comment]: https://github.com/tailscale/caddy-tailscale/issues/34#issuecomment-2145764893
+
 ### Running examples
 
 Multiple example configurations are provided in the [examples directory].