Add JAVA_AGENTS and JVM_AGENTS to load agents from URLs and paths (#4356)

docs/configuration/jvm-options.md

@@ -89,6 +89,33 @@ you can use
   JVM_DD_OPTS: fml.queryResult=confirm,name=value
 ```
 
+## Java and JVM Agents
+
+Java agents, such as [authlib-injector](https://github.com/yushijinhun/authlib-injector), can be loaded into the server by setting `JAVA_AGENTS`, and native JVM agents by setting `JVM_AGENTS`. Like [the `MODS` / `PLUGINS` variables](../mods-and-plugins/index.md#modsplugins-list), both are a comma or newline delimited list of
+
+- URL of an agent file
+- container path to an agent file
+- container path to a directory containing agent jar files for `JAVA_AGENTS`, or `.so` files for `JVM_AGENTS`
+
+The agents are copied into `/data/agents/java` and `/data/agents/jvm`, and each one is passed to the JVM as `-javaagent:<file>` or `-agentpath:<file>`, in file name order.
+
+!!! example
+
+    ```yaml
+        environment:
+          JAVA_AGENTS: |
+            https://www.example.com/agents/agent.jar
+            /agents
+    ```
+
+!!! note "Auto-removal"
+
+    Entries that are removed from the `JAVA_AGENTS` or `JVM_AGENTS` list will be automatically removed from the agents directory, so they are no longer loaded.
+
+!!! note "Agent options"
+
+    An agent that needs options, such as `-javaagent:agent.jar=options`, can still be passed with `JVM_OPTS` instead.
+
 ## Enable Remote JMX for Profiling
 
 To enable remote JMX, such as for profiling with VisualVM or JMC, set the environment variable `ENABLE_JMX` to "true", set `JMX_HOST` to the IP/host running the Docker container, and add a port forwarding of TCP port 7091, such as:

docs/variables.md

@@ -147,6 +147,18 @@ alternatively, you can mount: <code>/etc/localtime:/etc/localtime:ro
             <td><code></code></td>
             <td>⬜️</td>
         </tr>
+        <tr>
+            <td><code>JAVA_AGENTS</code></td>
+            <td>Comma or newline delimited list of Java agent jar URLs, container paths to jars, or container paths to directories of jars, which are loaded with <code>-javaagent</code></td>
+            <td><code></code></td>
+            <td>⬜️</td>
+        </tr>
+        <tr>
+            <td><code>JVM_AGENTS</code></td>
+            <td>Comma or newline delimited list of native JVM agent URLs, container paths to agents, or container paths to directories of <code>.so</code> agents, which are loaded with <code>-agentpath</code></td>
+            <td><code></code></td>
+            <td>⬜️</td>
+        </tr>
         <tr>
             <td><code>EXTRA_ARGS</code></td>
             <td>Arguments that would usually be passed to the jar file (those which are written after the filename)</td>