Updated docker image in compose to be all lowercase

[SethAngell]

Corrected error in docker-compose image which was leading to a failure to deploy

Description

Images in docker-compose must be all lowercase, however the DL in spotDL was not.
Updated the image to be all lowercase.

Related Issue

Addresses Issue #1616

Motivation and Context

I'm trying to make some passes on projects with easy fixes, so that primary contributors can focus on the important, domain specific stuff. This felt like a great mosquito task to knock out.

How Has This Been Tested?

Tested a full docker-compose spin up with updated file.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have read the CORE VALUES document
• I have added tests to cover my changes
• All new and existing tests passed