Add Docker support for Linux build testing

- Introduced a `Dockerfile` for setting up a minimal Ubuntu-based build environment with required dependencies.
- Added `docker-build.sh` script to simplify Linux build and test execution using Docker or Podman.
- Updated `DEVELOPER_GUIDE.md` with instructions for using Docker/Podman for Linux builds, including CI/CD integration examples.

tests/test_migration_coverage.cc

@@ -424,7 +424,7 @@ TEST (Migration_SyntaxHighlighter_Pattern)
 			break; // Should never happen
 		}
 		std::string line = buf.GetLineString(row);
-		EXPECT_TRUE(line.size() >= 0); // Always true, but validates access
+		// Successfully accessed line - size() is always valid for std::string
 	}
 }