ci: update docker; no need for TLS in internal env

https://gitlab.com/gitlab-org/gitlab-runner/issues/4501

.gitlab-ci.yml

@@ -19,6 +19,8 @@ variables:
   PIPENV_VENV_IN_PROJECT: "true"
   PIPENV_CACHE_DIR: ".cache/"
 
+  # No need for TLS because everything runs on the same machine
+  DOCKER_TLS_CERTDIR: ""
 stages:
   - environment
   - prebuild

ci/environment.yml

@@ -1,6 +1,6 @@
 environment:
   stage: environment
-  image: docker:18.09.6
+  image: docker
   variables:
     GIT_SUBMODULE_STRATEGY: none  # no need to fetch submodules
     CONTAINER_NAME: "$CI_REGISTRY/trezor/trezor-firmware/environment"