Prepare: run E2E smoketests with GitHub actions (#23301)

- Port E2E testing scripts from cypress-ui-automation
- Move server to docker-compose, move E2E images to ecrpublic
* Integrate General channel renaming, fixes
- Add local automation-dashboard
- Add readme
- Add E2E smoketests
- Bump postgres to 12

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Saturnino Abril <saturnino.abril@gmail.com>
Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

e2e-tests/README.md

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+# E2E testing for the Mattermost web client
+
+This directory contains the E2E testing code for the Mattermost web client.
+
+### How to run locally
+
+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:
+1. 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
+2. (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
+3. `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
+4. `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. Modifting 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 dasbhoard later, you can run `make clean-env-placeholders` to remove references to the local dasbhoard (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.