Add missing `yum clean all` calls to keep the images smaller

centos7-32bit-self-contained/Dockerfile

@@ -21,4 +21,5 @@ RUN yum update -y && \
         nasm \
         qt-devel \
         qt5-qtbase-devel \
-        which
+        which && \
+    yum clean all

centos7-32bit-system-dependencies/Dockerfile

@@ -17,4 +17,5 @@ RUN yum update -y && \
         patch \
         texlive-collection-latex \
         texlive-dvipng \
-        which
+        which && \
+    yum clean all

centos7-self-contained/Dockerfile

@@ -20,4 +20,5 @@ RUN yum update -y && \
         qt5-qtbase-devel \
         which && \
     yum install -y centos-release-scl && \
-    yum install -y devtoolset-8
+    yum install -y devtoolset-8 && \
+    yum clean all

centos7-system-dependencies/Dockerfile

@@ -16,4 +16,5 @@ RUN yum update -y && \
         texlive-dvipng \
         which && \
     yum install -y centos-release-scl && \
-    yum install -y devtoolset-8
+    yum install -y devtoolset-8 && \
+    yum clean all