[CI] Added drone config and fixed badge url in README

---

kind: 'pipeline'

name: 'build'

type: 'kubernetes'

steps:

# build and publish

- image: 'docker.io/plugins/docker'

  name: 'publish'

  pull: 'always'

  settings:

    # registery and repos

    registry: 'cr.simoncor.net'

    repo: 'cr.simoncor.net/siempie/roundcube'

    mtu: '1440'

    # build stuff

    dockerfile: 'Dockerfile.linux.amd64'

    daemon_off: 'false'

    # authentication

    username:

      from_secret: 'docker_username'

    password:

      from_secret: 'docker_password'

    # tags

    tags:

    - 'v1'

    - 'latest'

[![Build Status](https://ci.simoncor.net/api/badges/siempie/hugo-sws/status.svg)](https://ci.simoncor.net/siempie/hugo-sws)

[![Build Status](https://ci.simoncor.net/api/badges/siempie/roundcube/status.svg)](https://ci.simoncor.net/siempie/roundcube)

Simple container with Roundcube and plugins pre-installed.

| package | version | link |