Add feature flags in config.toml

What does this MR do?

1. refactor(ff): change default type to bool
2. Allow user to set a feature flag inside of the config.toml with [runners.feature_flags] which aren't overridable able by an job variables.

Why was this MR needed?

1. refactor(ff): change default type to bool

   Having a bool as a string makes it hard when you want to check if pragmatically. It's easier to transform a bool into a string than vice versa.

2. feat(ff): set feature flag in config.toml

   In !2805 (merged) we want to change how docker+machine executor creates machines and we want to roll this out behind a feature flag. At the moment users can only specify a feature flag as a job variable, which won't work in this case because a machine is created before a job exists so there is no way we can set this.

What's the best way to test this MR?

Feature flag defined in config.toml

1. Start a docker executor with the following config.toml

   [[runners]]
     name = "docker"
     url = "http://gdk.test:3000/"
     token = ""
     executor = "docker"
     limit = 2
     [runners.feature_flags]
       "FF_NETWORK_PER_BUILD" = true
     [runners.docker]
       image = "alpine:3.12"
       privileged = true

2. Start a job

   .gitlab-ci.yml

   variables:
     SLEEP: 60
   
   job:
     stage: test
     script:
       - sleep ${SLEEP}

3. Whilst the job is running make sure that a network is created for the build

   docker network ls

   $ docker network ls
   NETWORK ID     NAME                                                       DRIVER    SCOPE
   9f60f6e85862   bridge                                                     bridge    local
   a8d215125ce7   host                                                       host      local
   d071437ccf78   minikube                                                   bridge    local
   44a1a7d5bd1a   none                                                       null      local
   7897d6060eb2   runner-4b72igtx-project-20-concurrent-0-job-2195-network   bridge    local
   b478d99ffde3   spamcheck_default                                          bridge    local

   job log

   

Feature flag turned off in config.toml but turned on inside of envionrments

1. Start a docker executor with the following config.toml

   config.toml

   [[runners]]
     name = "docker"
     url = "http://gdk.test:3000/"
     token = ""
     executor = "docker"
     environment = ["FF_NETWORK_PER_BUILD=true"]
     [runners.feature_flags]
       "FF_NETWORK_PER_BUILD" = false
     [runners.docker]
       image = "alpine:3.12"
       privileged = true

2. Start a job

   .gitlab-ci.yml

   variables:
     SLEEP: 60
   
   job:
     stage: test
     script:
       - sleep ${SLEEP}

3. Whilst running the job make sure that a network wasn't created since we disable it in [runners.feature_flags]

   docker network ls

   # No network created for the build!
   $ docker network ls
   NETWORK ID     NAME                DRIVER    SCOPE
   9f60f6e85862   bridge              bridge    local
   a8d215125ce7   host                host      local
   d071437ccf78   minikube            bridge    local
   44a1a7d5bd1a   none                null      local
   b478d99ffde3   spamcheck_default   bridge    local

   job log

   

Feature flag turned off in config.toml but turned on inside of job variables

1. Start a docker executor with the following config.toml

   config.toml

   [[runners]]
     name = "docker"
     url = "http://gdk.test:3000/"
     token = ""
     executor = "docker"
     [runners.feature_flags]
       "FF_NETWORK_PER_BUILD" = false
     [runners.docker]
       image = "alpine:3.12"
       privileged = true

2. Start a job

   .gitlab-ci.yml

   variables:
     SLEEP: 60
     FF_NETWORK_PER_BUILD: "true"
   
   job:
     stage: test
     script:
       - sleep ${SLEEP}

3. Whilst running the job make sure that a network wasn't created since we disable it in [runners.feature_flags]

   docker network ls

   # No network created for the build!
   $ docker network ls
   NETWORK ID     NAME                DRIVER    SCOPE
   9f60f6e85862   bridge              bridge    local
   a8d215125ce7   host                host      local
   d071437ccf78   minikube            bridge    local
   44a1a7d5bd1a   none                null      local
   b478d99ffde3   spamcheck_default   bridge    local

   job log

   

What are the relevant issue numbers?

To safely roll out #27613 (closed)