Add support for packwiz toml definitions (#1163)

README.md

@@ -61,6 +61,7 @@ By default, the container will download the latest version of the "vanilla" [Min
       * [Modpack data directory](#modpack-data-directory)
       * [Buggy start scripts](#buggy-start-scripts)
       * [Fixing "unable to launch forgemodloader"](#fixing-unable-to-launch-forgemodloader)
+   * [Running a server with a packwiz modpack](#running-a-server-with-a-packwiz-modpack)
    * [Working with mods and plugins](#working-with-mods-and-plugins)
       * [Optional plugins, mods, and config attach points](#optional-plugins-mods-and-config-attach-points)
       * [Auto-downloading SpigotMC/Bukkit/PaperMC plugins](#auto-downloading-spigotmcbukkitpapermc-plugins)
@@ -660,6 +661,20 @@ then you apply a workaround by adding this to the run invocation:
 
     -e FTB_LEGACYJAVAFIXER=true
 
+## Running a server with a packwiz modpack
+
+[packwiz](https://packwiz.infra.link/) is a CLI tool for maintaining and providing modpack definitions, with support for both CurseForge and Modrinth as sources. See the [packwiz tutorial](https://packwiz.infra.link/tutorials/getting-started/) for more information.
+
+To configure server mods using a packwiz modpack, set the `PACKWIZ_URL` environment variable to the location of your `pack.toml` modpack definition:
+
+    docker run -d -v /path/on/host:/data -e TYPE=FABRIC \
+        -e "PACKWIZ_URL=https://example.com/modpack/pack.toml" \
+        itzg/minecraft-server
+
+packwiz modpack defitions are processed before other mod definitions (`MODPACK`, `MODS`, etc.) to allow for additional processing/overrides you may want to perform (in case of mods not available via Modrinth/CurseForge, or you do not maintain the pack).
+
+> packwiz is pre-configured to only download server mods. If client-side mods are downloaded and cause issues, check your pack.toml configuration, and make sure any client-only mods are not set to `"both"`, but rather `"client"` for the side configuration item.
+
 ## Working with mods and plugins
 
 ### Optional plugins, mods, and config attach points