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/server.start.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+set -e -u -o pipefail
+cd $(dirname $0)
+. .e2erc
+
+export BRANCH_DEFAULT=$(git branch --show-current)
+export BRANCH=${BRANCH:-$BRANCH_DEFAULT}
+export BUILD_ID_DEFAULT=$(date +%s)
+export BUILD_ID=${BUILD_ID:-$BUILD_ID_DEFAULT}
+export CI_BASE_URL="${CI_BASE_URL:-localhost}"
+
+# Cleanup old containers, if any
+mme2e_log "Stopping leftover E2E containers, if any are running"
+${MME2E_DC_SERVER} down -v
+
+# Generate .env.server
+mme2e_log "Generating .env.server"
+mme2e_generate_envfile_from_var_names >.env.server <<EOF
+MM_LICENSE
+EOF
+envarr=$(echo ${MM_ENV:-} | tr "," "\n")
+for env in $envarr; do
+  echo "$env" >> .env.server
+done
+
+# Generate .env.cypress
+mme2e_log "Generating .env.cypress"
+mme2e_generate_envfile_from_var_names >.env.cypress <<EOF
+BRANCH
+BUILD_ID
+CI_BASE_URL
+AUTOMATION_DASHBOARD_URL
+AUTOMATION_DASHBOARD_TOKEN
+EOF
+
+# Launch mattermost-server, and wait for it to be healthy
+mme2e_log "Waiting for server image to be available"
+mme2e_wait_image ${SERVER_IMAGE} 30 60
+mme2e_log "Starting E2E containers"
+${MME2E_DC_SERVER} up -d
+if ! mme2e_wait_service_healthy server 60 10; then
+  mme2e_log "Mattermost container not healthy, retry attempts exhausted. Giving up." >&2
+  exit 1
+fi
+mme2e_log "Mattermost container is running and healthy"