[Snyk] Security upgrade jupyterhub/jupyterhub from 1.3 to 1.5

[snyk-bot]

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• Dockerfile

We recommend upgrading to jupyterhub/jupyterhub:1.5, as this image has only 84 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	614	NULL Pointer Dereference SNYK-UBUNTU2004-OPENSSL-1049132	No Known Exploit
	614	NULL Pointer Dereference SNYK-UBUNTU2004-OPENSSL-1089071	No Known Exploit
	614	Buffer Overflow SNYK-UBUNTU2004-OPENSSL-1569470	No Known Exploit
	733	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-UBUNTU2004-OPENSSL-2426343	No Known Exploit
	514	CVE-2021-3995 SNYK-UBUNTU2004-UTILLINUX-2387728	No Known Exploit

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