diff --git a/src/ebd_toolchain/main.py b/src/ebd_toolchain/main.py
index 8b15ab1..ed2cd68 100644
--- a/src/ebd_toolchain/main.py
+++ b/src/ebd_toolchain/main.py
@@ -48,7 +48,7 @@
 from rebdhuhn.models.errors import GraphConversionError, PlantumlConversionError, SvgConversionError
 from rebdhuhn.plantuml import convert_graph_to_plantuml
 
-from ebd_toolchain.ahb_pruefi import download_ahb_db, get_ebd_to_pruefis_mapping
+from .ahb_pruefi import download_ahb_db, get_ebd_to_pruefis_mapping
 
 _logger = logging.getLogger(__name__)
 
diff --git a/unittests/test_no_absolute_intra_imports.py b/unittests/test_no_absolute_intra_imports.py
new file mode 100644
index 0000000..e22167f
--- /dev/null
+++ b/unittests/test_no_absolute_intra_imports.py
@@ -0,0 +1,39 @@
+"""
+Guard against absolute intra-package imports in ebd_toolchain.
+
+The Dockerfile runs main.py directly via `python ebd_toolchain/main.py`, which means
+ebd_toolchain's parent directory is NOT on sys.path. Absolute imports like
+`from ebd_toolchain.ahb_pruefi import ...` fail with ModuleNotFoundError in Docker.
+Relative imports (`from .ahb_pruefi import ...`) work in both contexts.
+"""
+
+import ast
+from pathlib import Path
+
+_PACKAGE_DIR = Path(__file__).parent.parent / "src" / "ebd_toolchain"
+
+
+def _is_absolute_intra_package(module_name: str) -> bool:
+    return module_name == "ebd_toolchain" or module_name.startswith("ebd_toolchain.")
+
+
+def test_no_absolute_intra_package_imports() -> None:
+    """Ensure all intra-package imports use relative syntax."""
+    violations: list[str] = []
+    for py_file in _PACKAGE_DIR.rglob("*.py"):
+        tree = ast.parse(py_file.read_text(encoding="utf-8"))
+        for node in ast.walk(tree):
+            if isinstance(node, ast.ImportFrom) and node.module and _is_absolute_intra_package(node.module):
+                violations.append(f"{py_file.name}:{node.lineno} — `from {node.module} import ...` should be relative")
+            elif isinstance(node, ast.Import):
+                for alias in node.names:
+                    if _is_absolute_intra_package(alias.name):
+                        violations.append(
+                            f"{py_file.name}:{node.lineno} — `import {alias.name}` should be a relative import"
+                        )
+
+    assert not violations, (
+        "Absolute intra-package imports break the Docker build "
+        "(main.py is run directly, so ebd_toolchain's parent is not on sys.path).\n"
+        "Use relative imports instead (e.g. `from .ahb_pruefi import ...`).\n\n" + "\n".join(violations)
+    )