fix(security): ignore developer-local environment files

.dockerignore

@@ -8,3 +8,6 @@
 **/*.swp
 **/*.swo
 **/.DS_Store
+.env
+.env.*
+!.env.example

.gitignore

@@ -33,6 +33,11 @@ hs_err_pid*
 node_modules/
 package-lock.json
 
+# Developer-local environment files
+.env
+.env.*
+!.env.example
+
 # Temporary files
 *.tmp
 *.bak

etl-service/src/test/java/com/xtrmetl/etl/config/RepositorySecretFilePolicyTest.java

@@ -0,0 +1,63 @@
+package com.xtrmetl.etl.config;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Guards repository boundaries that keep developer-local environment files out of Git and Docker
+ * build contexts.
+ *
+ * <p>The local Compose guidance uses environment overrides for database and service credentials.
+ * Those files must remain untracked and excluded from Docker build transfer by default, while a
+ * reviewed non-secret {@code .env.example} template may remain available for source-controlled
+ * documentation.</p>
+ */
+class RepositorySecretFilePolicyTest {
+
+    @Test
+    void localEnvironmentFilesAreIgnoredWhileExampleTemplateMayBeTracked() throws IOException {
+        Path repositoryRoot = findRepositoryRoot(Path.of("").toAbsolutePath().normalize());
+        assertNotNull(repositoryRoot, "repository root containing .gitignore must be discoverable");
+
+        assertEnvironmentFileRules(
+                Files.readAllLines(repositoryRoot.resolve(".gitignore")),
+                "root .gitignore"
+        );
+    }
+
+    @Test
+    void localEnvironmentFilesAreExcludedFromDockerBuildContext() throws IOException {
+        Path repositoryRoot = findRepositoryRoot(Path.of("").toAbsolutePath().normalize());
+        assertNotNull(repositoryRoot, "repository root containing .gitignore must be discoverable");
+
+        Path dockerIgnore = repositoryRoot.resolve(".dockerignore");
+        assertTrue(Files.isRegularFile(dockerIgnore), "root .dockerignore must exist");
+        assertEnvironmentFileRules(Files.readAllLines(dockerIgnore), "root .dockerignore");
+    }
+
+    private static void assertEnvironmentFileRules(List<String> patterns, String boundary) {
+        assertTrue(patterns.contains(".env"), boundary + " must ignore .env");
+        assertTrue(patterns.contains(".env.*"), boundary + " must ignore environment-specific .env files");
+        assertTrue(
+                patterns.contains("!.env.example"),
+                boundary + " must permit a reviewed non-secret .env.example template"
+        );
+    }
+
+    private static Path findRepositoryRoot(Path start) {
+        Path candidate = start;
+        while (candidate != null) {
+            if (Files.isRegularFile(candidate.resolve(".gitignore"))) {
+                return candidate;
+            }
+            candidate = candidate.getParent();
+        }
+        return null;
+    }
+}