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/.ci/dashboard.override.yml

@@ -0,0 +1,24 @@
+---
+version: '3.1'
+
+services:
+    dashboard:
+        image: node:16.17.0
+        environment:
+          PG_URI: postgres://mmuser:mostest@db:5432/automation_dashboard_db
+          JWT_SECRET: s8gGBA3ujKRohSw1L8HLOY7Jjnu2ZYv8 # Generated with e.g. `dd if=/dev/urandom count=24 bs=1 2>/dev/null | base64 -w0`
+          JWT_USER: cypress-test
+          JWT_ROLE: integration
+          JWT_ALG: HS256
+          JWT_EXPIRES_IN: 365d
+          ALLOWED_USER: cypress-test
+          ALLOWED_ROLE: integration
+        user: "${MME2E_UID}"
+        volumes:
+          - "../:/app"
+          - "../../dashboard.entrypoint.sh:/usr/local/bin/dashboard.entrypoint.sh:ro"
+          - "../../.env.dashboard:/var/local/.env.dashboard:rw"
+        working_dir: /app
+        entrypoint: /usr/local/bin/dashboard.entrypoint.sh
+        ports:
+            - 4000:4000