docs: clarify Testcontainers environment variable usage with Gradle

- Updated Spring plugin README and user guide to explain how to handle Testcontainers runtime environment variables with the Gradle JVM.
- Added examples for using `--no-daemon` or stopping existing Gradle daemons to ensure environment variables are recognized.

Author: CoderYellow

doc/user-guide.adoc

@@ -1557,6 +1557,13 @@ By default, the plugin wires `bigDataTestStart` before `JavaExec` and `Test` tas
 GRADLE_USER_HOME=/data/.gradle ./gradlew bigDataTestRun
 ----
 
+Testcontainers runtime environment variables, such as `TESTCONTAINERS_RYUK_DISABLED`, must be visible to the Gradle JVM that runs the kit. If a Gradle daemon is already running, a one-off shell prefix might not be visible to that daemon. Use `--no-daemon`, or stop existing daemons before running the task:
+
+[source,bash]
+----
+TESTCONTAINERS_RYUK_DISABLED=true GRADLE_USER_HOME=/data/.gradle ./gradlew --no-daemon bigDataTestRun
+----
+
 Disable automatic task wiring when you want explicit task dependencies:
 
 [source,kotlin]

example/spring-gradle-plugin/README.md

@@ -22,6 +22,12 @@ For manual troubleshooting without starting the Spring app, keep the containers
 GRADLE_USER_HOME=/data/.gradle ../../gradlew bigDataTestRun
 ```
 
+For Testcontainers runtime environment variables, make sure the Gradle JVM sees them. With a reused Gradle daemon, a one-off shell prefix might not be picked up, so use `--no-daemon` or stop existing daemons first:
+
+```bash
+TESTCONTAINERS_RYUK_DISABLED=true GRADLE_USER_HOME=/data/.gradle ../../gradlew --no-daemon bigDataTestRun
+```
+
 The Gradle plugin starts Kerberos, HDFS, and LocalStack S3 before `bootRunLocal` launches the
 Spring JVM, runs the TOML extensions, and injects endpoint/extension output as JVM system
 properties and environment variables. The application reads the JVM properties from