Test against multiple JVM versions

This commit is a test commit for GitHub actions. Afterwards, these will
become comments and documentation.

.github/workflows/ci-batect-gradle.yml

@@ -12,9 +12,17 @@ on:
   workflow_dispatch:
 
 jobs:
+  # Does your code need to check against several JVM versions?
   build:
     name: Builds and tests for Gradle
     runs-on: ubuntu-latest
+    # First run in GitHub actions - verify this
+    # Afterwards, change to a comment
+    strategy:
+      matrix:
+        include:
+          - java: 17
+          - java: 21
 
     steps:
       - name: Clone repository

.github/workflows/ci-batect-maven.yml

@@ -15,6 +15,13 @@ jobs:
   build:
     name: Builds and tests for Maven
     runs-on: ubuntu-latest
+    # First run in GitHub actions - verify this
+    # Afterwards, change to a comment
+    strategy:
+      matrix:
+        include:
+          - java: 17
+          - java: 21
 
     steps:
       - name: Clone repository

.github/workflows/ci-earthly-gradle.yml

@@ -15,6 +15,13 @@ jobs:
   build:
     name: Builds and tests
     runs-on: ubuntu-latest
+    # First run in GitHub actions - verify this
+    # Afterwards, change to a comment
+    strategy:
+      matrix:
+        include:
+          - java: 17
+          - java: 21
 
     steps:
       - name: Use Earthly

.github/workflows/ci-earthly-maven.yml

@@ -15,6 +15,13 @@ jobs:
   build:
     name: Builds and tests
     runs-on: ubuntu-latest
+    # First run in GitHub actions - verify this
+    # Afterwards, change to a comment
+    strategy:
+      matrix:
+        include:
+          - java: 17
+          - java: 21
 
     steps:
       - name: Use Earthly

README.md

@@ -446,6 +446,10 @@ choice for the JDK.
   mismatching or confusing build paths, project configuration, and Gradle
   sorting it out. This is an issue for local-only builds; local builds using a
   container (such as via [_Batect_](#setup-local-ci)) lower these concerns
+* In GitHub Actions, building supports cross-checking multiple JVM versions,
+  use
+  [the `matrix` feature](https://docs.github.com/en/actions/using-jobs/using-a-matrix-for-your-jobs).
+  See [the example GitHub actions](./.github/workflows/ci-batect-maven.yml)
 
 ### Managing your Java environment