docker: prefer CMD to ENTRYPOINT

Specifying watchfrr as CMD instead of ENTRYPOINT allows one to easily
override this command when starting a docker container. This allows
simple, manual testing via (e.g.) bash. With ENTRYPOINT only the
container will simply explode with an exit code if watchfrr exits.

For instance one could start a shell session in this container via:

```
docker run --name test --rm -i -t <frr-container> bash
```

The default behavior (`docker run <frr-container>` with no command
specified) is not changed.

Signed-off-by: Wesley Coakley <wcoakley@nvidia.com>

docker/alpine/Dockerfile

@@ -56,4 +56,4 @@ RUN apk add \
 		--allow-untrusted /pkgs/apk/*/*.apk \
 	&& rm -rf /pkgs
 COPY docker/alpine/docker-start /usr/lib/frr/docker-start
-ENTRYPOINT [ "/sbin/tini", "--", "/usr/lib/frr/docker-start" ]
+CMD [ "/sbin/tini", "--", "/usr/lib/frr/docker-start" ]

docker/centos-7/Dockerfile

@@ -40,4 +40,4 @@ COPY --from=centos-7-builder /rpmbuild/RPMS/ /pkgs/rpm/
 RUN yum install -y /pkgs/rpm/*/*.rpm \
     && rm -rf /pkgs
 COPY docker/centos-7/docker-start /usr/lib/frr/docker-start
-ENTRYPOINT [ "/usr/lib/frr/docker-start" ]
+CMD [ "/usr/lib/frr/docker-start" ]

docker/centos-8/Dockerfile

@@ -41,4 +41,4 @@ COPY --from=centos-8-builder /rpmbuild/RPMS/ /pkgs/rpm/
 RUN yum install -y /pkgs/rpm/*/*.rpm \
     && rm -rf /pkgs
 COPY docker/centos-8/docker-start /usr/lib/frr/docker-start
-ENTRYPOINT [ "/usr/lib/frr/docker-start" ]
+CMD [ "/usr/lib/frr/docker-start" ]

docker/debian/Dockerfile

@@ -17,4 +17,4 @@ RUN apt-get update && \
     rm -rf /var/lib/apt/lists/*
 
 ADD docker-start /usr/sbin/docker-start
-ENTRYPOINT ["/usr/sbin/docker-start"]
+CMD ["/usr/sbin/docker-start"]

docker/debian/docker-start

@@ -7,4 +7,6 @@ set -e
 ##
 chown -R frr:frr /etc/frr
 /etc/init.d/frr start
-exec sleep 10000d
+
+# Sleep forever
+exec tail -f /dev/null

docker/ubuntu18-ci/Dockerfile

@@ -68,4 +68,4 @@ RUN cd ~/frr && \
 RUN cd ~/frr && make check || true
 
 COPY docker/ubuntu18-ci/docker-start /usr/sbin/docker-start
-ENTRYPOINT ["/usr/sbin/docker-start"]
+CMD ["/usr/sbin/docker-start"]

docker/ubuntu20-ci/Dockerfile

@@ -71,4 +71,4 @@ RUN cd ~/frr && \
 RUN cd ~/frr && make check || true
 
 COPY docker/ubuntu20-ci/docker-start /usr/sbin/docker-start
-ENTRYPOINT ["/usr/sbin/docker-start"]
+CMD ["/usr/sbin/docker-start"]