Try loging in manually.

2 jobs for master in 1 minute and 34 seconds (queued for 1 second)
Name Stage Failure
failed
build_TreeMaker_cmssw_standalone_CMSSW_10_2_21_dockerhub Build
-rw-rw-rw-    1 0        0            609 Apr  9 07:53 .gitlab-ci.yml
-rw-rw-rw- 1 0 0 2.0K Apr 9 07:53 Dockerfile
-rw-rw-rw- 1 0 0 2.6K Apr 9 07:53 Makefile
-rwxrwxrwx 1 0 0 229 Apr 9 07:53 entrypoint.sh
$ echo "Test logging into the registry"
Test logging into the registry
$ docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
/busybox/sh: eval: line 107: docker: not found
ERROR: Job failed: exit code 127
failed
build_TreeMaker_cmssw_standalone_CMSSW_10_2_21 Build
-rw-rw-rw-    1 0        0            609 Apr  9 07:53 .gitlab-ci.yml
-rw-rw-rw- 1 0 0 2.0K Apr 9 07:53 Dockerfile
-rw-rw-rw- 1 0 0 2.6K Apr 9 07:53 Makefile
-rwxrwxrwx 1 0 0 229 Apr 9 07:53 entrypoint.sh
$ echo "Test logging into the registry"
Test logging into the registry
$ docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
/busybox/sh: eval: line 103: docker: not found
ERROR: Job failed: exit code 127