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.start.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+set -e -u -o pipefail
+cd $(dirname $0)
+. .e2erc
+
+MME2E_DASHBOARD_REF_DEFAULT="origin/main"
+MME2E_DASHBOARD_REF=${MME2E_DASHBOARD_REF:-$MME2E_DASHBOARD_REF_DEFAULT}
+
+mme2e_log "Cloning the automation-dashboard project"
+[ -d dashboard ] || git clone https://github.com/saturninoabril/automation-dashboard.git dashboard
+git -C dashboard fetch
+git -C dashboard checkout $MME2E_DASHBOARD_REF
+
+mme2e_log "Starting the dashboard"
+${MME2E_DC_DASHBOARD} up -d db dashboard
+
+mme2e_log "Generating the dashboard's local URL"
+MME2E_DC_DASHBOARD_NETWORK=$(${MME2E_DC_DASHBOARD} ps -q dashboard | xargs -l docker inspect | jq -r '.[0].NetworkSettings.Networks | (keys|.[0])')
+MME2E_DC_DASHBOARD_GATEWAY=$(docker network inspect $MME2E_DC_DASHBOARD_NETWORK | jq -r '.[0].IPAM.Config[0].Gateway')
+AUTOMATION_DASHBOARD_URL="http://${MME2E_DC_DASHBOARD_GATEWAY}:4000/api"
+
+mme2e_log "Generating a signed JWT token for accessing the dashboard"
+AUTOMATION_DASHBOARD_TOKEN=$(${MME2E_DC_DASHBOARD} exec -T -u $MME2E_UID dashboard node script/sign.js | awk '{ print $2; }') # The token secret is specified in the dashboard.override.yml file
+
+mme2e_log "Generating the .env.dashboard file, to point Cypress to the dashboard URL"
+mme2e_generate_envfile_from_var_names >.env.dashboard <<EOF
+AUTOMATION_DASHBOARD_URL
+AUTOMATION_DASHBOARD_TOKEN
+EOF