Documentation enhancements (#207)

This PR addresses the issue listed in issue #170 regarding how to set up rootless Docker. It also expands on the documentation to show how to create deployments for different environments.

Co-authored-by: ccureau <ccureau@noreply.gitea.io>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/207
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: Chris Cureau <cmcureau@gmail.com>
Co-committed-by: Chris Cureau <cmcureau@gmail.com>

examples/vm/README.md

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+## `act_runner` on Virtual or Physical Servers
+
+Files in this directory:
+
+- [`rootless-docker.md`](rootless-docker.md)
+  How to set up a rootless docker implementation of the runner.

examples/vm/rootless-docker.md

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+## Using Rootless Docker with`act_runner`
+
+Here is a simple example of how to set up `act_runner` with rootless Docker. It has been created with Debian, but other Linux should work the same way.
+
+Note: This procedure needs a real login shell -- using `sudo su` or other method of accessing the account will fail some of the steps below.
+
+As `root`:
+
+- Create a user to run both `docker` and `act_runner`. In this example, we use a non-privileged account called `rootless`.
+
+```bash
+ useradd -m rootless
+ passwd rootless
+```
+
+- Install [`docker-ce`](https://docs.docker.com/engine/install/)
+- (Recommended) Disable the system-wide Docker daemon
+
+     ``systemctl disable --now docker.service docker.socket``
+
+As the `rootless` user:
+
+- Follow the instructions for [enabling rootless mode](https://docs.docker.com/engine/security/rootless/)
+- Add the following lines to the `/home/rootless/.bashrc`:
+
+```bash
+ export XDG_RUNTIME_DIR=/home/rootless/.docker/run
+ export PATH=/home/rootless/bin:$PATH
+ export DOCKER_HOST=unix:///run/user/1001/docker.sock
+```
+
+- Reboot. Ensure that the Docker process is working.
+- Create a directory for saving `act_runner` data between restarts
+
+ `mkdir /home/rootless/act_runner`
+
+- Register the runner from the data directory
+
+```bash
+ cd /home/rootless/act_runner
+ act_runner register
+```
+
+- Generate a `act_runner` configuration file in the data directory. Edit the file to adjust for the system.
+
+```bash
+ act_runner generate-config >/home/rootless/act_runner/config
+```
+
+- Create a new user-level`systemd` unit file as `/home/rootless/.config/systemd/user/act_runner.service` with the following contents:
+
+```bash
+ Description=Gitea Actions runner
+ Documentation=https://gitea.com/gitea/act_runner
+ After=docker.service
+
+ [Service]
+ Environment=PATH=/home/rootless/bin:/sbin:/usr/sbin:/home/rootless/bin:/home/rootless/bin:/home/rootless/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games
+ Environment=DOCKER_HOST=unix:///run/user/1001/docker.sock
+ ExecStart=/usr/bin/act_runner daemon -c /home/rootless/act_runner/config
+ ExecReload=/bin/kill -s HUP $MAINPID
+ WorkingDirectory=/home/rootless/act_runner
+ TimeoutSec=0
+ RestartSec=2
+ Restart=always
+ StartLimitBurst=3
+ StartLimitInterval=60s
+ LimitNOFILE=infinity
+ LimitNPROC=infinity
+ LimitCORE=infinity
+ TasksMax=infinity
+ Delegate=yes
+ Type=notify
+ NotifyAccess=all
+ KillMode=mixed
+
+ [Install]
+ WantedBy=default.target
+```
+
+- Reboot
+
+After the system restarts, check that the`act_runner` is working and that the runner is connected to Gitea.
+
+````bash
+ systemctl --user status act_runner
+ journalctl --user -xeu act_runner