Docker: Add x11-apps to manual test installation

Running xeyes is a good check that X works.

Also, don't remove the package data base in the run setup, that makes
further configurations (too) inconvenient.

Change-Id: I5760b99f6e2ed3e4539580049f04d24b64cecfd8
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/manual/docker/Dockerfile-qt-5-ubuntu-20.04-build

@@ -34,6 +34,7 @@ RUN apt-get update && \
     nim \
     linux-tools-common \
     valgrind \
+    x11-apps \
  && apt-get clean
 
 # && rm -rf /var/lib/apt/lists/*

tests/manual/docker/Dockerfile-qt-5-ubuntu-20.04-clang-llbd-build

@@ -34,6 +34,7 @@ RUN apt-get update && \
     nim \
     linux-tools-common \
     valgrind \
+    x11-apps \
  && apt-get clean
 
 # && rm -rf /var/lib/apt/lists/*

tests/manual/docker/Dockerfile-qt-5-ubuntu-20.04-run

@@ -14,5 +14,7 @@ RUN apt-get update \
     gdb \
     linux-tools-common \
     valgrind \
- && apt-get clean \
- && rm -rf /var/lib/apt/lists/*
+    x11-apps \
+ && apt-get clean
+
+# && rm -rf /var/lib/apt/lists/*