replaced all md5sum.txt with md5sum.result

[Jeltje]

This makes the Docker image compatible with the GA4GH workflow examples, which expect <tool>.result instead of <tool>.txt

Did a test run

dockstore tool launch --local-entry Dockstore.cwl --json test.dockstore.json

using a local build with the updated Docker image (and the dockerPull line in Dockstore.cwl replaced with the test image) to verify this runs.

[Jeltje]

I believe the check fails because the system builds docker build -t quay.io/briandoconnor/dockstore-tool-md5sum but then pulls quay.io/briandoconnor/dockstore-tool-md5sum:1.0.3 from quay.io because that build is specified in the cwl file