diff --git a/pungi/phases/image_build.py b/pungi/phases/image_build.py
index cd034cd2..c28ccc2f 100644
--- a/pungi/phases/image_build.py
+++ b/pungi/phases/image_build.py
@@ -28,7 +28,7 @@ from productmd.rpms import Rpms
 # values. This dict is imported and extended for similar purposes by other
 # phases (at least osbuild and kiwibuild).
 EXTENSIONS = {
-    "docker": ["tar.gz", "tar.xz"],
+    "docker": ["tar.xz"],
     "liveimg-squashfs": ["liveimg.squashfs"],
     "qcow": ["qcow"],
     "qcow2": ["qcow2"],
diff --git a/tests/test_imagebuildphase.py b/tests/test_imagebuildphase.py
index 2cf982da..b3397f9a 100644
--- a/tests/test_imagebuildphase.py
+++ b/tests/test_imagebuildphase.py
@@ -843,12 +843,12 @@ class TestCreateImageBuildThread(PungiTestCase):
             "amd64": [
                 "/koji/task/1235/tdl-amd64.xml",
                 "/koji/task/1235/Fedora-Docker-Base-20160103.amd64.qcow2",
-                "/koji/task/1235/Fedora-Docker-Base-20160103.amd64.tar.gz",
+                "/koji/task/1235/Fedora-Docker-Base-20160103.amd64.tar.xz",
             ],
             "x86_64": [
                 "/koji/task/1235/tdl-x86_64.xml",
                 "/koji/task/1235/Fedora-Docker-Base-20160103.x86_64.qcow2",
-                "/koji/task/1235/Fedora-Docker-Base-20160103.x86_64.tar.gz",
+                "/koji/task/1235/Fedora-Docker-Base-20160103.x86_64.tar.xz",
             ],
         }
 
@@ -893,9 +893,9 @@ class TestCreateImageBuildThread(PungiTestCase):
                     link_type="hardlink-or-copy",
                 ),
                 mock.call.link(
-                    "/koji/task/1235/Fedora-Docker-Base-20160103.amd64.tar.gz",
+                    "/koji/task/1235/Fedora-Docker-Base-20160103.amd64.tar.xz",
                     self.topdir
-                    + "/compose/Client/amd64/images/Fedora-Docker-Base-20160103.amd64.tar.gz",  # noqa: E501
+                    + "/compose/Client/amd64/images/Fedora-Docker-Base-20160103.amd64.tar.xz",  # noqa: E501
                     link_type="hardlink-or-copy",
                 ),
                 mock.call.link(
@@ -905,9 +905,9 @@ class TestCreateImageBuildThread(PungiTestCase):
                     link_type="hardlink-or-copy",
                 ),
                 mock.call.link(
-                    "/koji/task/1235/Fedora-Docker-Base-20160103.x86_64.tar.gz",
+                    "/koji/task/1235/Fedora-Docker-Base-20160103.x86_64.tar.xz",
                     self.topdir
-                    + "/compose/Client/x86_64/images/Fedora-Docker-Base-20160103.x86_64.tar.gz",  # noqa: E501
+                    + "/compose/Client/x86_64/images/Fedora-Docker-Base-20160103.x86_64.tar.xz",  # noqa: E501
                     link_type="hardlink-or-copy",
                 ),
             ],
@@ -919,8 +919,8 @@ class TestCreateImageBuildThread(PungiTestCase):
                 "type": "qcow2",
                 "arch": "amd64",
             },
-            "image_dir/Client/amd64/Fedora-Docker-Base-20160103.amd64.tar.gz": {
-                "format": "tar.gz",
+            "image_dir/Client/amd64/Fedora-Docker-Base-20160103.amd64.tar.xz": {
+                "format": "tar.xz",
                 "type": "docker",
                 "arch": "amd64",
             },
@@ -929,8 +929,8 @@ class TestCreateImageBuildThread(PungiTestCase):
                 "type": "qcow2",
                 "arch": "x86_64",
             },
-            "image_dir/Client/x86_64/Fedora-Docker-Base-20160103.x86_64.tar.gz": {
-                "format": "tar.gz",
+            "image_dir/Client/x86_64/Fedora-Docker-Base-20160103.x86_64.tar.xz": {
+                "format": "tar.xz",
                 "type": "docker",
                 "arch": "x86_64",
             },