packaging: add support for CentOS 8

The resulting executable has not been tested in detail yet.
I ran 'hg version' and 'hg clone', which worked fine
(except for extensions acting up due to Python 3).

Differential Revision: https://phab.mercurial-scm.org/D7216

contrib/packaging/docker/centos8

@@ -0,0 +1,15 b''
+FROM centos:centos8
+
+RUN groupadd -g %GID% build && \
+    useradd -u %UID% -g %GID% -s /bin/bash -d /build -m build
+
+RUN yum install -y \
+	gcc \
+	gettext \
+	make \
+	python3-devel \
+	python3-docutils \
+	rpm-build
+
+# For creating repo meta data
+RUN yum install -y createrepo

Makefile

@@ -183,10 +183,12 b' packaging_targets := \\'
   centos5 \
   centos6 \
   centos7 \
+  centos8 \
   deb \
   docker-centos5 \
   docker-centos6 \
   docker-centos7 \
+  docker-centos8 \
   docker-debian-jessie \
   docker-debian-stretch \
   docker-fedora \

contrib/packaging/Makefile

@@ -16,7 +16,8 b' FEDORA_RELEASE := 31'
 CENTOS_RELEASES := \
   5 \
   6 \
-  7
+  7 \
+  8
 
 # Build a Python for these CentOS releases.
 CENTOS_WITH_PYTHON_RELEASES := 5 6