Add Cleanroom server type (#4300)

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

@@ -0,0 +1,67 @@
+A [Cleanroom server](https://cleanroommc.com/) can be automatically downloaded, upgraded, and run by setting the environment variable `TYPE` to "CLEANROOM".
+
+!!! example
+
+    ```shell
+    docker run -e TYPE=CLEANROOM ...
+    ```
+    
+    or in a compose file
+    ```yaml
+        environment:
+          TYPE: CLEANROOM
+    ```
+
+Cleanroom is a loader specifically for Minecraft 1.12.2, so `VERSION` will be ignored. By default, the latest version of Cleanroom will be selected. You can also choose a specific Cleanroom version by setting `CLEANROOM_LOADER_VERSION` with that version, such as "0.7.0".
+
+!!! example
+
+    ```shell
+    docker run -e TYPE=CLEANROOM -e CLEANROOM_LOADER_VERSION=0.7.0 ...
+    ```
+    
+    or in a compose file
+    ```yaml
+        environment:
+          TYPE: CLEANROOM
+          CLEANROOM_LOADER_VERSION: "0.7.0"
+    ```
+
+By default, the latest version of the installer will be selected. You can instead choose a specific installer version by setting `CLEANROOM_INSTALLER_VERSION`, such as "0.2.0".
+
+!!! example
+
+    ```shell
+    docker run -e TYPE=CLEANROOM -e CLEANROOM_INSTALLER_VERSION=0.2.0 ...
+    ```
+    
+    or in a compose file
+    ```yaml
+        environment:
+          TYPE: CLEANROOM
+          CLEANROOM_INSTALLER_VERSION: "0.2.0"
+    ```
+
+!!! note
+
+    You do not need to setup this unless required. The installer come from Cleanroom support multi-version installation compare to legacy installer.
+
+To use a pre-downloaded Cleanroom installer, place it in a location mounted into the container and specify the container path with `CLEANROOM_FROM_FILE`. To download a Cleanroom installer from a custom location, such as your own file repository, specify the URL with `CLEANROOM_FROM_URL`.
+
+In both of the cases above, there is no need for the `CLEANROOM_LOADER_VERSION` or `CLEANROOM_INSTALLER_VERSION` variables.
+
+!!! note
+
+    A legacy installer is supported as-is, but issues will not be addressed.
+
+!!! note
+
+    If an error occurs while installing Cleanroom, it might be possible to resolve by temporarily setting `CLEANROOM_FORCE_REINSTALL` to "true". Be sure to remove that variable after successfully starting the server.
+
+Specified installer version is configurable via environment variables:
+
+- `CLEANROOM_INSTALLER_VERSION`: used to specific installer version, default is latest
+
+URLs configurable via environment variables:
+
+- `CLEANROOM_MAVEN_URL`: default is https://maven.cleanroommc.com