[container] Update Dockerfile for PEP 518 build

tests/distros/Dockerfile.centos

@@ -1,10 +1,18 @@
-FROM centos:7
+# syntax=docker/dockerfile:1.3
+
+FROM docker.io/library/centos:7
+
+RUN yum install -y epel-release && yum install -y python36 && yum clean all
 
-RUN yum install -y epel-release
-RUN yum install -y python34
 WORKDIR /usr/src/app
-COPY asciinema asciinema
-COPY tests tests
-ENV LANG en_US.utf8
-ENV SHELL /bin/bash
-ENV USER docker
+
+COPY asciinema/ asciinema/
+COPY tests/ tests/
+
+ENV LANG="en_US.utf8"
+
+USER nobody
+
+ENTRYPOINT ["/bin/bash"]
+
+# vim:ft=dockerfile