The E2E testing scripts depend on the following tools being installed on your system: docker, docker-compose, make, git, jq, and some common utilities (coreutils, findutils, bash, awk, sed, grep)

Instructions, tl;dr: create a local branch with your E2E test changes, then open a PR to the mattermost-server repo targeting the master branch (so that CI will produce the image that docker-compose needs), then run make in this directory.

Instructions, detailed:

Create the .ci/env file, and populate the variables you need, keeping in mind that:

The following variables will be passed over to the server container: MM_LICENSE (no enterprise features will be available if this is unset), and the exploded MM_ENV (a comma-separated list of env var specifications)
The following variables will be passed over to the cypress container: BRANCH, BUILD_ID, CI_BASE_URL, AUTOMATION_DASHBOARD_URL and AUTOMATION_DASHBOARD_TOKEN
The SERVER_IMAGE variable can also be set, if you want to select a custom mattermost-server image
The TEST_FILTER variable can also be set, to customize which tests you want cypress to run
All variables are optional, and will be set to sane defaults

(optional) make start-dashboard: start the automation-dashboard in the background

This also sets the AUTOMATION_DASHBOARD_URL and AUTOMATION_DASHBOARD_TOKEN variables for the cypress container
Note that if you run the dashboard locally, but also specify other AUTOMATION_DASHBOARD_* variables in your env, the latter variables will take precedence

make: start and prepare the server, then run the cypress tests

You can track the progress of the run in the http://localhost:4000/cycles dashboard, if you launched it locally

make stop: tears down the server (and the dashboard, if running), then cleans up the env placeholder files

Notes:

Aside from some exceptions (e.g. TEST_FILTER), most of the variables in .ci/env must be set before the make start-server command is run. Modifying that file afterwards has no effect, because the containers' env files are generated in that step.
If you restart the dashboard at any point, you must also restart the server containers, so that it picks up the new IP of the dashboard from the newly generated .env.dashboard file
If you started the dashboard locally in the past, but want to point to another dashboard later, you can run make clean-env-placeholders to remove references to the local dashboard (you'll likely need to restart the server)
Dynamically set variables for the server or cypress should be managed within the .env.* files, rather than in the docker-compose files, to streamline their management.

How to control which tests to run

The TEST_FILTER variable will control which test files to run Cypress tests against. Please check the e2e-tests/cypress/run_tests.js file for details about its format.