Since commit 5f71eac06e the Sphinx tool is required
to build the rST documentation.

This fixes:

 $ ./configure --enable-docs

 ERROR: User requested feature docs
        configure was not able to find it.
        Install texinfo, Perl/perl-podlators and python-sphinx

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
v2: Only include base image (the ones to cross-build qemu,
    not the ones to cross-build user-space tests) (Alex)
---
 tests/docker/dockerfiles/debian10.docker | 1 +
 tests/docker/dockerfiles/debian9.docker  | 1 +
 2 files changed, 2 insertions(+)

diff --git a/tests/docker/dockerfiles/debian10.docker b/tests/docker/dockerfiles/debian10.docker
index aeeb151b52..cf57d8218f 100644
--- a/tests/docker/dockerfiles/debian10.docker
+++ b/tests/docker/dockerfiles/debian10.docker
@@ -27,5 +27,6 @@ RUN DEBIAN_FRONTEND=noninteractive eatmydata \
         pkg-config \
         psmisc \
         python \
+        python3-sphinx \
         texinfo \
         $(apt-get -s build-dep qemu | egrep ^Inst | fgrep '[all]' | cut -d\  -f2)
diff --git a/tests/docker/dockerfiles/debian9.docker b/tests/docker/dockerfiles/debian9.docker
index 5f23a35404..52c589a7d6 100644
--- a/tests/docker/dockerfiles/debian9.docker
+++ b/tests/docker/dockerfiles/debian9.docker
@@ -27,5 +27,6 @@ RUN DEBIAN_FRONTEND=noninteractive eatmydata \
         pkg-config \
         psmisc \
         python \
+        python3-sphinx \
         texinfo \
         $(apt-get -s build-dep qemu | egrep ^Inst | fgrep '[all]' | cut -d\  -f2)
-- 
2.20.1