Clear live apt indexes before snapshot install

ober

5ad57b98725528e7cba860ae828f0d1c3a0dcf4f

diff --git a/Dockerfile.qt b/Dockerfile.qt
index f74302e..489f588 100644
--- a/Dockerfile.qt
+++ b/Dockerfile.qt
@@ -30,6 +30,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates
       -e "s#https?://(archive|security)\\.ubuntu\\.com/ubuntu/?#https://snapshot.ubuntu.com/ubuntu/${APT_SNAPSHOT}/#g" \
       -e "s#https?://ports\\.ubuntu\\.com/ubuntu-ports/?#https://snapshot.ubuntu.com/ubuntu/${APT_SNAPSHOT}/#g" \
       /etc/apt/sources.list.d/ubuntu.sources && \
+    rm -rf /var/lib/apt/lists/* && \
     apt-get update && apt-get install -y --no-install-recommends --allow-downgrades \
     bash \
     ca-certificates \