Allow service alias from config in Docker executor

Description

Some people may need to alias a service within the Runner config to make it easier to enforce service includes.

Proposal

Allow alias tags to be included for services within the Runner's config.toml file.

We need to define a new toml table array so users can define something like below. Having the table above, we then in the future extend this so that he user can set entrypoint & command in a follow up issues, no no breaking changes.

[[runners]]
  name = "xxx"
  url = "xxx"
  token = "xxx"
  executor = "shell"
  shell = "powershell"
  [[runners.docker.services]]
     name = "postgres:11"
     alias = "db"
  [[runners.docker.services]]
     name = "redis:latest"
     alias = "cache"

With this, we would also need to deprecate the docker.service configuration we already have in place, which will be removed in a future major release.

Since !1476 (merged) we have a specific struct that we can try and reuse fo configuration.

This is for customer: https://gitlab.my.salesforce.com/00161000004zoBW

Deprecation

The old syntax services = ['postgres', 'mysql]` is deprecated and going to be removed in %13.0 !1673 (merged)