DOCKER_AUTH_CONFIG doesn't work with multiple registries
## Summary
Authentication settings for private registries are not applied when pulling a base image for the job if DOCKER_AUTH_CONFIG has more than one registry.
## Steps to reproduce
1. Add DOCKER_AUTH_CONFIG variable to Project-Settings-CI/CD
`{
"auths": {
"<first_registry>": {
"auth": "<first_base64_token>"
},
"<second_registry>": {
"auth": "<second_base64_token>"
}
}
}`
<details>
<summary> .gitlab-ci.yml </summary>
```yml
stages:
- first
- second
first:
stage: first
image: first_registry/image
script:
echo $DOCKER_AUTH_CONFIG
pwd
tags:
- run_docker
second:
stage: second
image: second_registry/image
script:
echo $DOCKER_AUTH_CONFIG
pwd
tags:
- run_docker
```
</details>
## Actual behavior
Job can't pull base image and fails after 2 retries.
## Expected behavior
Base image is pulled and executed
## Relevant logs and/or screenshots
<details>
<summary> job log </summary>
```sh
WARNING: Failed to pull image with policy "always": Error response from daemon: unauthorized: access to the requested resource is not authorized (manager.go:205:0s)
ERROR: Preparation failed: failed to pull image "first_registry/image" with specified policies [always]: Error response from daemon: unauthorized: access to the requested resource is not authorized (manager.go:205:0s)
```
</details>
## Environment description
We use shared runners with Docker 20.10.7
<details>
<summary> config.toml contents (removed proxy-related variables) </summary>
```toml
[[runners]]
name = "run01_docker"
pre_build_script = "[ ! -f $HOME/.docker/config.json ] && [ -w $HOME ] && ( mkdir -p $HOME/.docker/ && echo \"{ \\\"proxies\\\": { \\\"default\\\":
{ \\\"httpProxy\\\": \\\"$http_proxy\\\", \\\"httpsProxy\\\": \\\"$https_proxy\\\", \\\"noProxy\\\": \\\"docker:2375,docker:2376,$no_proxy\\\" }
} }\" > $HOME/.docker/config.json )"
output_limit = 8192
executor = "docker"
environment = ["DOCKER_DRIVER=overlay2"]
[runners.custom_build_dir]
[runners.docker]
tls_verify = false
image = "ubuntu:xenial"
privileged = true
disable_entrypoint_overwrite = false
oom_kill_disable = false
disable_cache = false
volumes = ["/cache", "/var/run/docker.sock:/var/run/docker.sock", "/home/coinop/.docker:/root/.docker", "/home/coinop/.m2:/home/coinop/.m2", "/home/coinop/.gradle:/home/coinop/.gradle"]
shm_size = 0
[runners.cache]
[runners.cache.s3]
[runners.cache.gcs]
```
</details>
### Used GitLab Runner version
```
Running with gitlab-runner 13.12.0 (7a6612da)
on run01_docker XPyy_c8y
Preparing the "docker" executor
```
-->
## Possible fixes
issue