Add docker-compose.yml and document how to run the site in Docker.

Part of #534

[ci skip]

Author: php-coder

docker/README.md

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+## Run MyStamps in Docker
+
+MyStamps application can be run as a Docker container. While there are many
+possible configurations, we support only those that are typically used:
+
+* **a single container** where the application is running with `test` profile.
+  In this mode, it also runs in-memory database H2. This is roughly the same as
+  executing `mvn spring-boot:run` command but inside a Docker container.
+
+  * Build a WAR file that will be used in container building:
+    ```console
+    $ mvn package
+    $ cd docker
+    $ ln -v ../target/mystamps.war mystamps.war
+    ```
+    By the way, don't forget to remove this hard link after using.
+  * Build an image and run a container with `docker-compose`:
+    ```console
+    $ docker-compose up -d
+    ```
+  * The same but with a plain Docker:
+    ```console
+    $ docker build -t mystamps:latest .
+    $ docker run -d -p 8080:8080 mystamps:latest
+    ```
+

docker/docker-compose.yml

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+version: '3'
+
+services:
+  web:
+    build: ./
+    ports: [ "8080:8080" ]