change version from 16.04 to 18.04

[awkj]

No description provided.

[kingdonb]

It's also here

<a name="upgrading_os"></a>
### Upgrading the operating system inside the container

Baseimage-docker images contain an Ubuntu 16.04 operating system.

... in README.md -- it looks like this text is not present in the Chinese versions.

Please update the PR and we'll merge it. Thanks!

[kingdonb]

Line 318 of README.md also has a 16.04 that needs to be changed

[kingdonb]

It looks like the CI might have lost the dockerhub creds again:

The command "env NAME=${NAME} VERSION=${VERSION} ./test/runner.sh" exited with 0.
$ docker login -u "${DOCKER_USERNAME}" -p "${DOCKER_PASSWORD}"; docker push ${NAME}:${VERSION};
Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com to create one.
Username: 

[PikachuEXE]

I think there are 2 places on each readme doc that mention the version of ubuntu
So still "Changes requested"

[Theaxiom]

LGTM! 👍