Add examples for several features

Signed-off-by: aiordache <anca.iordache@docker.com>

docs/examples.md

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+# Compose file samples - ECS specific
+
+
+
+## Service 
+
+A service mapping may define a Docker image and runtime constraints and container requirements.
+
+```yaml
+services:
+  test:
+    image: "image"
+    command: "command"
+    entrypoint: "entrypoint"
+    environment:
+      - "FOO=BAR"
+    cap_add:
+      - SYS_PTRACE
+    cap_drop:
+      - SYSLOG
+    init: true
+    user: "user"
+    working_dir: "working_dir"
+```
+
+
+###### Task size
+
+Set resource limits that will get translated to Fargate task size values:
+
+```yaml
+services:
+  test:
+    image: nginx
+    deploy:
+      resources:
+        limits:
+          cpus: '0.5'
+          memory: 2048M
+```
+
+###### Logging
+Pass options to awslogs driver
+```yaml
+services:
+  foo:
+    image: nginx
+    logging:
+      options:
+        awslogs-datetime-pattern: "FOO"
+
+x-aws-logs_retention: 10
+```
+
+
+###### Autoscaling
+
+Set a CPU percent target
+```yaml
+services:
+  foo:
+    image: nginx
+    deploy:
+      x-aws-autoscaling: 75
+```
+
+
+###### GPU
+Set `generic_resources` for services that require accelerators as GPUs.
+```yaml
+services:
+  learning:
+    image: tensorflow/tensorflow:latest-gpus
+    deploy:
+      resources:
+        reservations:
+          memory: 32Gb
+          cpus: "32"
+          generic_resources:
+          - discrete_resource_spec:
+            kind: gpus
+            value: 2
+```
+
+
+
+
+#### Load Balancers
+
+When a service in the compose file exposes a port, a load balancer is being created and configured to distribute the traffic between all containers.
+
+There are 2 types of Load Balancers that can be created. For a service exposing a non-http port/protocol, a __Network Load Balancer (NLB)__ is created. Services with http/https ports/protocols get an __Application Load Balancer (ALB)__.
+
+ There is only one load balancer created/configured for a Compose stack. If there are both http/non-http ports configured for services in a compose stack, an NLB is created.
+
+The compose file below configured only the http port,therefore, on deployment it gets an ALB created.
+
+```yaml
+services:
+  app:
+    image: nginx
+    ports:
+      - 80:80
+```
+NLB is created for non-http port
+```yaml
+services:
+  app:
+    image: nginx
+    ports:
+      - 8080:8080
+```
+
+To use the http protocol with custom ports and get an ALB, use the `x-aws-protocol` port property.
+```yaml
+services:
+  test:
+    image: nginx
+    ports:
+      - target: 8080
+        x-aws-protocol: http
+```
+
+To re-use an external load balancer and avoid creating a dedicated one, set the top-level property `x-aws-loadbalancer` as below:
+```yaml
+x-aws-loadbalancer: "LoadBalancerName"
+services:
+  app:
+    image: nginx
+    ports:
+      - 80:80
+```
+
+Similarly, an external `VPC` and `Cluster` can be reused:
+
+```yaml
+x-aws-vpc: "vpc-25435e"
+x-aws-cluster: "ClusterName"
+
+services:
+  app:
+    image: nginx
+    ports:
+      - 80:80
+```
+
+Keep in mind, that external resources are not managed as part of the compose stack's lifecycle.
+
+
+## Volumes
+
+```yaml
+services:
+  app:
+    image: nginx
+    volumes:
+      - data:/test
+volumes:
+  data:
+```
+To use of an external volume that has been previously created, set its id/ARN as the name:
+
+```yaml
+services:
+  app:
+    image: nginx
+    volumes:
+      - data:/test
+
+volumes:
+  data:
+    external: true
+    name: "fs-f534645"
+```
+
+Customize volume configuration via `driver_opts`
+
+```yaml
+services:
+  test:
+    image: nginx
+volumes:
+  db-data: 
+    driver_opts:
+        backup_policy: ENABLED
+        lifecycle_policy: AFTER_30_DAYS
+        performance_mode: maxIO
+        throughput_mode: provisioned
+        provisioned_throughput: 1024
+```
+
+
+## Secrets
+Secrets are stored in __AWS SecretsManager__ as strings and are mounted to containers  under `/run/secrets/`.
+```yaml
+services:
+  app:
+    image: nginx
+    ports:
+      - 80:80
+    secrets:
+      - mysecret
+
+secrets:
+  mysecret: 
+    file: ./secrets/mysecret.txt
+```
+
+When using external secrets, set a valid secret `ARN` under the `name` property:
+
+```yaml
+services:
+  app:
+    image: nginx
+    secrets:
+      - foo_bar
+
+secrets:
+  foo_bar:
+    name: "arn:aws:secretsmanager:eu-west-3:xxx:secret:foo_bar"
+    external: true
+```
+
+## Access private images
+When a service is configured with an image from a private repository on Docker Hub, make sure you have configured pull credentials correctly before deploying the Compose stack.
+
+To create a pull credential, create a file with the following content:
+```sh
+$ cat creds.json
+{
+  "username":"DockerHubID",
+  "password":"GeneratedHubTokenOrPassword"
+}
+```
+To create the pull credential and retrieve the `ARN/ID` to use in the compose file run:
+```sh
+$ docker secret create pullcred /path/to/creds.json
+arn:aws:secretsmanager:eu-west-3:xxx:secret:pullcred
+```
+
+Use the `ARN` in the output to set the `x-aws-pull_credentials` service property as below:
+```yaml
+services:
+  app:
+    image: DockerHubID/privateimage
+    x-aws-pull_credentials: arn:aws:secretsmanager:eu-west-3:xxx:secret:pullcred
+    ports:
+      - 80:80
+```
+
+
+
+
+#### Networks
+
+Networks are mapped to security groups.
+```yaml
+services:
+  test:
+    image: nginx
+networks:
+  default:
+```
+Using an external network/security group:
+```yaml
+services:
+  test:
+    image: nginx
+networks:
+  default:
+    external: true
+    name: sg-123abc
+```
+