(v3.1.2) - Sinergym Dockerfile fix

Description

As mentioned in https://github.com/ugr-sail/sinergym/issues/376, there was a problem when installing Sinergym using Docker container and VS Code. This PR solves this issue.

Files were not being copied on the correct path. This resulted on source code changes not taking effect on experiments run inside the container.

Paths have been corrected and Sinergym version is updated.

[x] I have raised an issue to propose this change (required for new features and bug fixes)

Why is this change required? What problem does it solve? Please, reference issue or issues opened previously.

Fixes #376

[x] 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)
[ ] Documentation (update in the documentation)
[ ] Improvement (of an existing feature)
[ ] Others

[x] I've read the CONTRIBUTION guide (required)
[ ] My change requires a change to the documentation.
[ ] I have updated the tests.
[ ] I have updated the documentation accordingly.
[ ] I have reformatted the code using autopep8 second level aggressive.
[ ] I have reformatted the code using isort.
[ ] I have ensured cd docs && make spelling && make html pass (required if documentation has been updated.)
[x] I have ensured pytest tests/ -vv pass. (required).
[x] I have ensured pytype -d import-error sinergym/ pass. (required)