Add unstable warning

Co-authored-by: Julian Groß  <[email protected]>

README.md

@@ -29,7 +29,7 @@ A workaround exists, please see [here](https://github.com/myoung34/docker-github
 | ubuntu focal | `x86_64`,`arm64` | `/\d\.\d{3}\.\d+/` | [latest](https://hub.docker.com/r/myoung34/github-runner/tags?page=1&name=latest) | This is the latest build (Rebuilt nightly and on master merges). Tags without an OS name are included. | Tags without an OS name *before* 9/17/2020 are `eoan`. `armv7` support stopped 9/18/2020 due to inconsistent docker-ce packaging |
 | ubuntu bionic | `x86_64`,`armv7`,`arm64` | `/\d\.\d{3}\.\d+-ubuntu-bionic/` | [ubuntu-bionic](https://hub.docker.com/r/myoung34/github-runner/tags?page=1&name=ubuntu-bionic) | This is the latest build from bionic (Rebuilt nightly and on master merges). Tags with `-ubuntu-bionic` are included and created on [upstream tags](https://github.com/actions/runner/tags). | |
 | debian buster | `x86_64`,`arm64` |  `/\d\.\d{3}\.\d+-debian-buster/` | [debian-buster](https://hub.docker.com/r/myoung34/github-runner/tags?page=1&name=debian-buster) | This is the latest build from buster (Rebuilt nightly and on master merges). Tags with `-debian-buster` are included and created on [upstream tags](https://github.com/actions/runner/tags). | |
-| debian sid | `x86_64`,`arm64` |  `/\d\.\d{3}\.\d+-debian-sid/` | [debian-sid](https://hub.docker.com/r/myoung34/github-runner/tags?page=1&name=debian-sid) | This is the latest build from sid (Rebuilt nightly and on master merges). Tags with `-debian-sid` are included and created on [upstream tags](https://github.com/actions/runner/tags). | |
+| debian sid | `x86_64`,`arm64` |  `/\d\.\d{3}\.\d+-debian-sid/` | [debian-sid](https://hub.docker.com/r/myoung34/github-runner/tags?page=1&name=debian-sid) | This is the latest build from sid (Rebuilt nightly and on master merges). Tags with `-debian-sid` are included and created on [upstream tags](https://github.com/actions/runner/tags). | Debian sid is considered unstable by Debian. |
 | ubuntu xenial | `x86_64`,`arm64` |  `/\d\.\d{3}\.\d+-ubuntu-xenial/` | [ubuntu-xenial](https://hub.docker.com/r/myoung34/github-runner/tags?page=1&name=ubuntu-xenial) | This is the latest build from xenial (Rebuilt nightly and on master merges). Tags with `-ubuntu-xenial` are included and created on [upstream tags](https://github.com/actions/runner/tags). | This is deprecated as of 7/15/2021 and will no longer receive tags. |
 
 These containers are built via Github actions that [copy the dockerfile](https://github.com/myoung34/docker-github-actions-runner/blob/master/.github/workflows/deploy.yml#L47), changing the `FROM` and building to provide simplicity.