docker: use HTTPS git URL for virglrenderer

When you clone the repository without previous commit history, 'git://'
doesn't protect from man-in-the-middle attacks.  HTTPS is more secure
since the client verifies the server certificate.

Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Suggested-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Tested-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Message-id: 20181108111531.30671-9-stefanha@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

tests/docker/dockerfiles/debian-amd64.docker

@@ -24,7 +24,7 @@ RUN DEBIAN_FRONTEND=noninteractive eatmydata \
         libegl1-mesa-dev \
         libepoxy-dev \
         libgbm-dev
-RUN git clone git://anongit.freedesktop.org/virglrenderer /usr/src/virglrenderer
+RUN git clone https://anongit.freedesktop.org/git/virglrenderer.git /usr/src/virglrenderer
 RUN cd /usr/src/virglrenderer && ./autogen.sh && ./configure --with-glx --disable-tests && make install
 
 # netmap