Bump nbconvert from 5.6.0 to 6.5.1 in /docker

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 5.6.0 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@5.6.0...6.5.1)

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

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

Author: dependabot[bot]

docker/requirements.txt

@@ -16,7 +16,7 @@ jupyterlab==1.2.4
 jupyterlab-server==1.0.6
 MarkupSafe==1.1.1
 mistune==0.8.3
-nbconvert==5.6.0
+nbconvert==6.5.1
 nbformat==4.4.0
 notebook==5.7.8
 pandocfilters==1.4.1