[Docker] Switched image format and added PostgreSQL service

[mnocon]

JIRA: https://jira.ez.no/browse/EZP-31620

This PR is the result of work done recently in docker-php, mostly ezsystems/docker-php#52.

I have:

• switched the default PHP_IMAGE to ezsystems/php:7.4-v2-node10 and dropped PHP_IMAGE_DEV, it's no longer used in our Docker stack
• added ENABLE_XDEBUG and XDEBUG_CONFIG variables to base-dev and base-prod (which should make Switched base-dev.yml docker-compose configuration for app container to -dev image #587 not needed anymore - xdebug is present in all images now)
• added a new PostgreSQL Docker service
• added a job running Behat tests on PostgreSQL (requires [Behat] Fixed tests running on PostgreSQL ezplatform-admin-ui#1445 to pass)

Passing PostgreSQL build: https://travis-ci.org/github/ezsystems/ezplatform/builds/715567661

[mnocon]

dev image has been merged with the base image, so the mysql client is present in PHP_IMAGE

[ViniTou]

I am still not sure if we should use 7.4 as default and not the lowest supported version.

[andrerom]

For defaults we should pick what we recommend, needs for test matrix to make sure we do pass on lowest and highest supported version should be set using ENVs in CI system IMO

[mnocon]

I've changed it to use PHP7.3 by defult (recommended for Platform 2.5). Travis will run on PHP7.1 and 7.4 for testing.

[andrerom]

Besides inline comments db-stack.yml is using hardcoded mariadb version, unsure why.
Maybe @vidarl remembers as he added it as-is 3 years ago.

[vidarl]

I think the reason for that was simply that I only tested docker stack on mariadb... But feel free to generalize it so it can be used with both, or rename it db-stack-mariadb.yml and make a db-stack-postgreql.yml counterpart if you think that is better

[mnocon]

I've decided to keep db-stack as it is, as I haven't used Docker Stack yet and I'm not sure how to approach it (and renaming it to db-stack-mariadb.yml without creating a PostgreSQL alternative would just break existing scripts without any benefit for people).

This PR in its current state allows us to run tests on PostgreSQL on Travis, which was my initial goal - IMHO we can work on adding more PostgreSQL support in the future if needed.