fix(CI): Force cypress v3.8.3 on the Docker Workflow (#336)

Closes #333.

## What does this PR do / solve?

Forcing cypress version 3.8.3 on the Docker Workflow with npx

.github/workflows/docker.yml

@@ -33,6 +33,6 @@ jobs:
 
       - name: Run Cypress E2E tests
         run: |
-          npx cypress run --config pluginsFile=false # because plugins/index.js requires typescript, which is not installed in this production image
+          npx cypress@3.8.3 run --config pluginsFile=false # because plugins/index.js requires typescript, which is not installed in this production image
 
 # To publish the Docker image from here, see https://github.com/openwhyd/openwhyd/pull/308/commits/1eacaa98885789642ba0073c9bb4d822021f0d95#diff-12a86cef0c4707531fdbabac3e38cb2aR36