Bump jupyterlab from 1.2.4 to 1.2.21 in /docker

Bumps [jupyterlab](https://github.com/jupyterlab/jupyterlab) from 1.2.4 to 1.2.21.
- [Release notes](https://github.com/jupyterlab/jupyterlab/releases)
- [Changelog](https://github.com/jupyterlab/jupyterlab/blob/master/CHANGELOG.md)
- [Commits](https://github.com/jupyterlab/jupyterlab/commits)

---
updated-dependencies:
- dependency-name: jupyterlab
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docker/requirements.txt

@@ -12,7 +12,7 @@ json5==0.8.5
 jsonschema==3.2.0
 jupyter-client==5.3.3
 jupyter-core==4.11.2
-jupyterlab==1.2.4
+jupyterlab==1.2.21
 jupyterlab-server==1.0.6
 MarkupSafe==1.1.1
 mistune==0.8.3