Build: allow dynamically change docker image (#18112)

* Build: allow dynamically change docker image

Right now, only for couple DBs.
With the 2.1, you can do `${TAG:-5.6}` without needing the `.env` file, but
it seems it would be better for back-compat to support `.env` right now.

Fixes #17891

* Build: add prefix for removal process

* Docs: mention makefile command instead of the bash

Since we are moving to makefile for things and make command
simplifies some of the actions you have to perform to work with
different environments

* Docs: mention dynamic tags in the docs

* Docs: avoid name collision

Fixes #16107

devenv/docker/blocks/mysql/.env

@@ -0,0 +1 @@
+mysql_version=5.6

devenv/docker/blocks/mysql/docker-compose.yaml

@@ -1,5 +1,5 @@
   mysql:
-    image: mysql:5.6
+    image: mysql:${mysql_version}
     environment:
       MYSQL_ROOT_PASSWORD: rootpass
       MYSQL_DATABASE: grafana

devenv/docker/blocks/postgres/.env

@@ -0,0 +1 @@
+postgres_version=9.3

devenv/docker/blocks/postgres/docker-compose.yaml

@@ -1,5 +1,5 @@
   postgrestest:
-    image: postgres:9.3
+    image: postgres:${postgres_version}
     environment:
       POSTGRES_USER: grafana
       POSTGRES_PASSWORD: password