[test coverage]: updating docker-compose.yml

[kennethrioja]

Summary of changes

• Simply adding ./coverage:/code/coverage to `docker-compose.yml

Motivation and context

Code coverage has been discussed here #1149 (comment) – in order to do proper code coverage while developing, we can now have a ./coverage folder for extensions to display test coverage in our favorite editor

For info I use VSCode and here are the steps to be able to see test coverage:

1. Downloaded the Coverage Gutters extension : https://marketplace.visualstudio.com/items?itemName=ryanluker.vscode-coverage-gutters
2. I added these lines in my user settings.json

  "coverage-gutters.lcovname": "coverage/lcov/code.lcov",
  "coverage-gutters.showLineCoverage": true,
  "coverage-gutters.showGutterCoverage": true,
  "coverage-gutters.coverageFileNames": [
    "coverage/lcov/code.lcov"
  ],
  "coverage-gutters.reloadOnChange": true,

3. Reloaded window (Cmd+Shift+P 'Reload Window'), and click on 'Watch' in the menu bar on the bottom of the editor window (or on Mac Cmd+Shift+8 to show, Cmd+Shift+9 to hide).
4. Enjoy!

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.