Replace the UBI7 base with Centos7.

I'm not sure why, but the build fails with an obscure message. For
future reference, th build error is "could not resolve digest for
registry.access.redhat.com/ubi7/ubi:latest" and in the dockerd log the
error is "transport: Error while dialing only one connection allowed".

Dockerfile

 # hadolint ignore=DL3006
-FROM registry.access.redhat.com/ubi7/ubi
+FROM registry.hub.docker.com/library/centos:7
 # hadolint ignore=DL3013,DL3033
-RUN yum install -y --disableplugin=subscription-manager \
-        autoconf \
-        automake \
-        binutils \
-        bison \
-        byacc \
-        cscope \
-        ctags \
-        diffstat \
-        doxygen \
-        elfutils \
-        flex \
-        gcc \
-        gcc-c++ \
-        gcc-gfortran \
-        gettext \
-        git \
-        indent \
-        intltool \
-        libtool \
-        make \
-        patch \
-        patchutils \
-        pkgconfig \
-        rcs \
-        redhat-rpm-config \
-        rh-python36-python-devel \
-        rh-python36-python-pip \
-        rpm-build \
-        rpm-sign \
-        subversion \
-        swig \
-        systemtap \
+RUN yum group install -y "Development tools" && \
+    yum install -y \
+        python3-devel \
+        python3-pip \
+        python3-wheel \
     && \
-    yum --disableplugin=subscription-manager clean all && \
-    pip install --no-cache-dir --progress-bar=off \
+    yum clean all && \
+    pip3 install --no-cache-dir --upgrade pip && \
+    pip3 install --no-cache-dir \
         docutils \
         pipenv \
         poetry \