Fix markdown formatting for docker instructions (#1616)

Signed-off-by: zhanluxianshen <[email protected]>

Author: zhanluxianshen

docker/slim/public-key-auth/instructions.md

@@ -1,32 +1,38 @@
 How to start a local Docker container with public key based authentication
 
 1. Configure Docker to use secrets in the build process
-
+```
 echo "DOCKER_BUILDKIT=1" >> ~/.docker/config.json
+```
 
-or edit the file manually to make sure it includes
-
+  or edit the file manually to make sure it includes
+```
 {
   "features": {
     "buildkit": true
   }
 }
+```
 
 2. Generate a public private key pair
-
+```
 mkdir -p ~/.ssh/runhouse/docker
 ssh-keygen -t rsa -b 4096 -C "[email protected]" -f ~/.ssh/runhouse/docker/id_rsa
+```
 
 3. The Dockerfile in the current directory should support public key based authentication using Docker Secrets for its build process
 
 4. Build the Docker container
-
+```
 docker build --no-cache --pull --rm -f "docker/slim/public-key-auth/Dockerfile" --secret id=ssh_key,src=$HOME/.ssh/runhouse/docker/id_rsa.pub -t runhouse:start .
+```
 
 5. Run the Docker container
-
+```
 docker run --rm --shm-size=4gb -it -p 32300:32300 -p 6379:6379 -p 52365:52365 -p 22:22 -p 443:443 -p 80:80 runhouse:start
+```
 
 6. Verify via SSH
-
+```
 ssh -i ~/.ssh/runhouse/docker/id_rsa rh-docker-user@localhost
+```