Fix badge image for maven build status (#164)

Co-authored-by: Jan Høydahl <janhoy@users.noreply.github.com>

We've intentionally overridden the requirement that commits be signed for this PR.

README.md

@@ -1,5 +1,5 @@
 ## A Docker image for the Cantaloupe IIIF image server
-[![Maven Build](https://github.com/uclalibrary/docker-cantaloupe/workflows/Maven%20Build%20for%20Snapshot%20or%20Release/badge.svg)](https://github.com/UCLALibrary/docker-cantaloupe/actions) [![Codacy Badge](https://app.codacy.com/project/badge/Grade/74e25a3c2515495aafb6b077fdde9a13)](https://www.codacy.com/gh/UCLALibrary/docker-cantaloupe?utm_source=github.com&utm_medium=referral&utm_content=UCLALibrary/docker-cantaloupe&utm_campaign=Badge_Grade) [![Known Vulnerabilities](https://snyk.io/test/github/uclalibrary/docker-cantaloupe/badge.svg)](https://snyk.io/test/github/uclalibrary/docker-cantaloupe)
+[![Maven Build](https://github.com/uclalibrary/docker-cantaloupe/actions/workflows/nightly.yml/badge.svg)](https://github.com/UCLALibrary/docker-cantaloupe/actions) [![Codacy Badge](https://app.codacy.com/project/badge/Grade/74e25a3c2515495aafb6b077fdde9a13)](https://www.codacy.com/gh/UCLALibrary/docker-cantaloupe?utm_source=github.com&utm_medium=referral&utm_content=UCLALibrary/docker-cantaloupe&utm_campaign=Badge_Grade) [![Known Vulnerabilities](https://snyk.io/test/github/uclalibrary/docker-cantaloupe/badge.svg)](https://snyk.io/test/github/uclalibrary/docker-cantaloupe)
 
 This project builds a Docker image for the [Cantaloupe IIIF image server](https://cantaloupe-project.github.io/cantaloupe). If you're interested in using a prebuilt image, you can find one on [UCLA Library's DockerHub account](https://hub.docker.com/repository/docker/uclalibrary/cantaloupe).