I am having this issue
system3:postgres saurabh-gupta2$ docker build -t postgres .
Sending build context to Docker daemon 38.91kB
Step 1/51 : FROM registry.access.redhat.com/rhel7/rhel
Get https://registry.access.redhat.com/v2/: Service Unavailable
docker run -t apline
Unable to find image 'apline:latest' locally
docker: Error response from daemon: Get https://registry-1.docker.io/v2/: Service Unavailable.
See 'docker run --help'.
I have looked for a solution that says to set proxy, but I have set the proxy for the wifi.
https://docs.docker.com/docker-for-mac/networking/#httphttps-proxy-support
Still, it is not working.
I have set proxy for docker too. It is not working.
in Preference -> proxies
Docker version 17.12 ce
I also want to know if the proxy is the issue then how can I check it is set, what is work around for this?

Here are few suggestions:
Check your network connections. For example by the following shell commands:
Check your proxy settings (e.g. in
/etc/default/docker).If above won't help, this could be a temporary issue with the Docker services (as per Service Unavailable).
Related: GH-842 - 503 Service Unavailable at http://hub.docker.com.
I had this problem for past days, it just worked after that.
You can consider raising the issue at
docker/hub-feedbackrepo, check at, Docker Community Forums, or contact Docker Support directly.