ci: remove unnecessary variables and services from CI config

Simplify the CI configuration by removing the unused DOCKER_HOST 
variable and the Docker service. This streamlines the build process 
and eliminates potential points of failure, enhancing overall reliability.

.gitlab-ci.yml

@@ -6,15 +6,10 @@ include:
 stages:
 - build
 
-variables:
-  DOCKER_HOST: tcp://docker:2375/
-
 image: buildtool/build-tools:${BUILDTOOLS_VERSION}
 
 build:
   stage: build
-  services:
-  - docker:${DOCKER_DIND_VERSION}
   script:
   - build
   - curl -Os https://uploader.codecov.io/latest/linux/codecov