fix docker mkdir error

Dockerfile.template

@@ -11,8 +11,9 @@ LABEL build-date-iso8601="${build_date}"
 
 WORKDIR /root
 
+# due https://github.com/moby/moby/issues/47021 we cannot have /root/.cache leftover as it causes random errors in CI
 RUN --mount=type=bind,source=${tar_path}/${tar_name},target=/tmp/${tar_name} \
-  pip install --no-cache-dir /tmp/${tar_name}[full]
+  pip install --no-cache-dir /tmp/${tar_name}[full] && rm -rf /root/.cache
 
 ENTRYPOINT ["b2"]
 CMD ["--help"]

changelog.d/+docker_mkdir_fail.infrastructure.md

@@ -0,0 +1 @@
+Fix CI failing on `mkdir` when testing docker image.

noxfile.py

@@ -609,11 +609,11 @@ def generate_dockerfile(session):
 
 def run_docker_tests(session, image_tag):
     """Run unittests against a docker image."""
+    docker_run_cmd = "docker run -i -v b2:/root/ -v /tmp:/tmp:rw --env-file ENVFILE"
     run_integration_test(
         session, [
             "--sut",
-            "docker run -i -v b2:/root -v /tmp:/tmp:rw "
-            f"--env-file ENVFILE {image_tag}",
+            f"{docker_run_cmd} {image_tag}",
             "--env-file-cmd-placeholder",
             "ENVFILE",
         ]
@@ -622,9 +622,7 @@ def run_docker_tests(session, image_tag):
         run_integration_test(
             session, [
                 "--sut",
-                "docker run -i -v b2:/root -v /tmp:/tmp:rw "
-                f"--entrypoint {binary_name} "
-                f"--env-file ENVFILE {image_tag}",
+                f"{docker_run_cmd} --entrypoint {binary_name} {image_tag}",
                 "--env-file-cmd-placeholder",
                 "ENVFILE",
             ]