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/Makefile

@@ -0,0 +1,28 @@
+.PHONY: all run stop
+all: run
+run: start-server prepare-server run-cypress
+stop: stop-server stop-dashboard clean-env-placeholders
+
+.PHONY: start-server prepare-server run-cypress stop-server restart-server
+start-server:
+	./.ci/server.start.sh
+prepare-server:
+	./.ci/server.prepare.sh
+run-cypress:
+	./.ci/server.run_cypress.sh
+stop-server:
+	./.ci/server.stop.sh
+restart-server: stop-server start-server
+
+.PHONY: start-dashboard stop-dashboard
+start-dashboard:
+	./.ci/dashboard.start.sh
+stop-dashboard:
+	./.ci/dashboard.stop.sh
+
+.PHONY: print-env-placeholders clean-env-placeholders
+print-env-placeholders:
+	find .ci -maxdepth 1 -name '.env.*'  | xargs --verbose -l cat
+clean-env-placeholders:
+	git reset .ci/.env.*
+	git checkout .ci/.env.*