Use "uv" for building virtual environments

Change-Id: I5e24e82e6347d351cac71255904b44aa558d449b
diff --git a/images/magnum/Dockerfile b/images/magnum/Dockerfile
index c2cb92c..724e7b8 100644
--- a/images/magnum/Dockerfile
+++ b/images/magnum/Dockerfile
@@ -14,8 +14,8 @@
 ARG MAGNUM_GIT_REF=db197e08a09da93062fc4222180051dadfc0f0d8
 ADD --keep-git-dir=true https://opendev.org/openstack/magnum.git#${MAGNUM_GIT_REF} /src/magnum
 RUN git -C /src/magnum fetch --unshallow
-RUN --mount=type=cache,mode=0755,target=/root/.cache/pip,sharing=private <<EOF bash -xe
-pip3 install \
+RUN --mount=type=cache,target=/root/.cache/uv <<EOF bash -xe
+uv pip install \
     --constraint /upper-constraints.txt \
         /src/magnum \
         magnum-cluster-api==0.24.2