Update README

Replace useless Gitlab CI badge with quay.io container image build
status.

Avoid using the word "docker", keep using it in command examples
however, as it is more generally known than "podman" for instance.

Signed-off-by: Nikola Forró <nforro@redhat.com>

README.md

@@ -1,6 +1,6 @@
 # Welcome to rebase-helper
 
-[![Travis CI build status](https://travis-ci.org/rebase-helper/rebase-helper.svg?branch=master)](https://travis-ci.org/rebase-helper/rebase-helper) [![Documentation build status](https://readthedocs.org/projects/rebase-helper/badge/?version=latest)](https://readthedocs.org/projects/rebase-helper) [![GitLab CI build status](https://gitlab.com/rebase-helper/rebase-helper/badges/master/build.svg)](https://gitlab.com/rebase-helper/rebase-helper/commits/master) [![PyPI version](https://img.shields.io/pypi/v/rebasehelper.svg)](https://pypi.org/project/rebasehelper) [![Project status](https://img.shields.io/pypi/status/rebasehelper.svg)](https://pypi.org/project/rebasehelper) [![Code Quality](https://api.codacy.com/project/badge/Grade/af059c941cd94f7aa557c3ae7ce75bb8)](https://www.codacy.com/app/rebase-helper/rebase-helper)
+[![Travis CI build status](https://travis-ci.org/rebase-helper/rebase-helper.svg?branch=master)](https://travis-ci.org/rebase-helper/rebase-helper) [![Documentation build status](https://readthedocs.org/projects/rebase-helper/badge/?version=latest)](https://readthedocs.org/projects/rebase-helper) [![Container image build status](https://quay.io/repository/rebasehelper/rebase-helper/status)](https://quay.io/repository/rebasehelper/rebase-helper) [![PyPI version](https://img.shields.io/pypi/v/rebasehelper.svg)](https://pypi.org/project/rebasehelper) [![Project status](https://img.shields.io/pypi/status/rebasehelper.svg)](https://pypi.org/project/rebasehelper) [![Code Quality](https://api.codacy.com/project/badge/Grade/af059c941cd94f7aa557c3ae7ce75bb8)](https://www.codacy.com/app/rebase-helper/rebase-helper)
 
 There are several steps that need to be done when rebasing a package. The goal of **rebase-helper** is to automate most of these steps.
 
@@ -29,7 +29,7 @@ Alternatively, you can run **rebase-helper** in a container:
 
 `$ docker run -it -e PACKAGE=foo rebasehelper/rebase-helper:latest`
 
-See [docker reference](https://rebase-helper.readthedocs.io/en/latest/user_guide/rebasing_in_container.html) for more information.
+See [container reference](https://rebase-helper.readthedocs.io/en/latest/user_guide/rebasing_in_container.html) for more information.
 
 ## What do I get from it?