update readme.md for all samples

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

samples/sparkjava-mysql/README.md

@@ -0,0 +1,70 @@
+## Compose sample application
+### Java Spark application with MySQL database
+
+Project structure:
+```
+.
+├── backend
+│   ├── Dockerfile
+│   └── ...
+├── db
+│   └── password.txt
+├── docker-compose.yaml
+└── README.md
+
+```
+
+[_docker-compose.yaml_](docker-compose.yaml)
+```
+version: "3.7"
+services:
+  backend:
+    build: backend
+    ports:
+    - 80:8080
+  db:
+    image: mysql:5.7
+    ...
+```
+The compose file defines an application with two services `backend` and `db`.
+When deploying the application, docker-compose maps port 8080 of the backend service container to port 80 of the host as specified in the file.
+Make sure port 80 on the host is not already being in use.
+
+## Deploy with docker-compose
+
+```
+$ docker-compose up -d
+Creating network "sparkjava-mysql_default" with the default driver
+Building backend
+...
+Successfully tagged sparkjava-mysql_backend:latest
+WARNING: Image for service backend was built because it did not already exist. To rebuild this image you must use `docker-compose build` or `docker-compose up --build`.
+Creating sparkjava-mysql_db_1      ... done
+Creating sparkjava-mysql_backend_1 ... done
+```
+
+## Expected result
+
+Listing containers must show two containers running and the port mapping as below:
+```
+$ docker ps
+CONTAINER ID        IMAGE                     COMMAND                  CREATED             STATUS              PORTS                  NAMES
+ee1e4f05d9f6        sparkjava-mysql_backend   "/bin/sh -c 'java -j…"   44 seconds ago      Up 43 seconds       0.0.0.0:80->8080/tcp   sparkjava-mysql_backend_1
+716025ddf65b        mysql:5.7                 "docker-entrypoint.s…"   44 seconds ago      Up 43 seconds       3306/tcp, 33060/tcp    sparkjava-mysql_db_1
+```
+
+After the application starts, run:
+```
+$ curl localhost:80
+["Blog post #0","Blog post #1","Blog post #2","Blog post #3","Blog post #4"]
+```
+
+Stop and remove the containers
+```
+$ docker-compose down
+Stopping sparkjava-mysql_backend_1 ... done
+Stopping sparkjava-mysql_db_1      ... done
+Removing sparkjava-mysql_backend_1 ... done
+Removing sparkjava-mysql_db_1      ... done
+Removing network sparkjava-mysql_default
+```

samples/sparkjava-mysql/docker-compose.yaml

@@ -2,7 +2,6 @@ version: "3.7"
 services:
   backend:
     build: backend
-    image: docker.io/docker/back
     ports:
     - 80:8080
     secrets: