Environment "leaks" into Docker container actions

Imagine a simple Docker container action, depending on the JAVA_HOME environment variable. It may be a simple Java app assembled with Gradle Application Plugin, which uses a script searching for Java VM in a JAVA_HOME.

But for the sake of simplicity in this repo we just print the environment, highlighting the JAVA_HOME, Java location (whereis java) and Java version:

#!/usr/bin/env sh

echo "Environment:"
printenv

printf "\n"
echo "JAVA_HOME:"
printenv JAVA_HOME

printf "\n"
echo "Java location:"
command -v java # openjdk images do not have `whereis`

printf "\n"
echo "Java version:"
java --version

Now, if one bundles this script into a Docker image and use it as an action, it will print something like this:

JAVA_HOME:
/usr/java/openjdk-17

Java location:
/usr/java/openjdk-17/bin/java

To reproduce the issue, run the actions/setup-java@v2 before this Docker container action:

- uses: actions/setup-java@v2
  with:
    distribution: 'adopt'
    java-version: '11'

- uses: madhead/actions-env-leak@main

The action will now print:

JAVA_HOME:
/opt/hostedtoolcache/Java_Adopt_jdk/11.0.11-9/x64

Java location:
/usr/java/openjdk-17/bin/java

Note that the JAVA_HOME environment variable now points to an invalid (inside the container) location. The action is broken!