refactor(ci): remove unused Docker variables from config

This change eliminates unnecessary Docker variable definitions from the
.gitlab-ci.yml file, simplifying the CI configuration and reducing
complexity in the build process. The removed variables were not in use
and did not contribute to the current build steps, streamlining the
configuration for better maintainability.

.gitlab-ci.yml

@@ -6,13 +6,6 @@ include:
 stages:
 - build
 
-variables:
-  DOCKER_HOST: tcp://docker:2376
-  DOCKER_TLS_CERTDIR: "/certs"
-  DOCKER_TLS_VERIFY: 1
-  DOCKER_CERT_PATH: "$DOCKER_TLS_CERTDIR/client"
-  DOCKER_DRIVER: overlay2
-
 image: buildtool/build-tools:${BUILDTOOLS_VERSION}
 
 build: