How do I find the Docker REST API URL?

Here are two approaches.

How do I access the Docker REST API remotely?

Warning: After this setup your Docker REST API port (in this case 1111) is exposed to remote access.

Here is how I enabled it on Ubuntu 16.04 (Xenial Xerus).

1. Edit the docker service file (it is better to avoid directly editing /lib/systemd/system/docker.service as it will be replaced on upgrades)

sudo systemctl edit docker.service

2. Add the following content

[Service]
ExecStart=
ExecStart=/usr/bin/docker daemon -H fd:// -H tcp://0.0.0.0:1111

For docker 18+, the content is a bit different:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock -H tcp://0.0.0.0:1111

3. Save the modified file. Here I used port 1111, but any free port can be used.

4. Make sure the Docker service notices the modified configuration:

systemctl daemon-reload

5. Restart the Docker service:

sudo service docker restart

6. Test

curl http://localhost:1111/version

7. See the result

{"Version":"17.05.0-ce","ApiVersion":"1.29","MinAPIVersion":"1.12","GitCommit":"89658be","GoVersion":"go1.7.5","Os":"linux","Arch":"amd64","KernelVersion":"4.15.0-20-generic","BuildTime":"2017-05-04T22:10:54.638119411+00:00"}

Now you can use the REST API.

How do I access the Docker REST API through a socket (from localhost)?

Connect the internal Unix socket somewhat like this,

Using curl

curl --unix-socket /var/run/docker.sock http:/localhost/version

And here is how to do it using PHP

$fs = fsockopen('/var/run/docker.sock');

fwrite($fs, "GET / HTTP/1.1\r\nHOST: http:/images/json\r\n\r\n");

while (!feof($fs)) {
    print fread($fs,256);
}

In PHP 7 you can use curl_setopt with the CURLOPT_UNIX_SOCKET_PATH option.

It depends on your host, but look for /etc/default/docker or /var/lib/boot2docker/profile (for Docker Machine hosts using a boot2docker VM).

You will see the port used by the docker daemon, for instance:

DOCKER_OPTS="-H unix:// -H tcp://0.0.0.0:2375"
                                        ^^^^^

Then get the IP address of the machine hosting your Docker daemon. (With a Docker Machine created host, that would be: docker-machine ip <yourmachine>.)

The URL to use is the combination of those the IP address and the port.

If you are on windows:

npipe:////./pipe/docker_engine

source: https://docs.docker.com/docker-for-windows/faqs/#how-do-i-connect-to-the-remote-docker-engine-api

If you are on Linux and need to connect to Docker API on the local machine, its URL is probably unix:///var/run/docker.sock, like it is mentioned in documentation: Develop with Docker Engine SDKs and API

By default the Docker daemon listens on unix:///var/run/docker.sock and the client must have root access to interact with the daemon. If a group named docker exists on your system, docker applies ownership of the socket to the group.

This might be helpful if you are connecting to Docker from a JetBrains IDE.