paper: suggest other release channel when build not found (#3000)

Author: itzg

Dockerfile

@@ -47,7 +47,7 @@ RUN easy-add --var os=${TARGETOS} --var arch=${TARGETARCH}${TARGETVARIANT} \
   --var version=${MC_SERVER_RUNNER_VERSION} --var app=mc-server-runner --file {{.app}} \
   --from ${GITHUB_BASEURL}/itzg/{{.app}}/releases/download/{{.version}}/{{.app}}_{{.version}}_{{.os}}_{{.arch}}.tar.gz
 
-ARG MC_HELPER_VERSION=1.39.8
+ARG MC_HELPER_VERSION=1.39.9
 ARG MC_HELPER_BASE_URL=${GITHUB_BASEURL}/itzg/mc-image-helper/releases/download/${MC_HELPER_VERSION}
 # used for cache busting local copy of mc-image-helper
 ARG MC_HELPER_REV=1

docs/types-and-platforms/server-types/paper.md

@@ -16,6 +16,14 @@ To allow for the selection of experimental builds, set `PAPER_CHANNEL` to "exper
     docker run ... -e TYPE=PAPER -e PAPER_CHANNEL=experimental ... 
     ```
 
+!!! tip
+
+    If you see the following error, it likely means you need to set the env var `PAPER_CHANNEL` to "experimental"
+    
+    ```
+    No build found for version 1.21 with channel 'default'
+    ```
+
 If you are hosting your own copy of Paper you can override the download URL with `PAPER_DOWNLOAD_URL=<url>`.
 
 If you have attached a host directory to the `/data` volume, then you can install plugins via the `plugins` subdirectory. You can also [attach a `/plugins` volume](../../mods-and-plugins/index.md#optional-plugins-mods-and-config-attach-points). If you add plugins while the container is running, you'll need to restart it to pick those up.