Add .dockerignore

Exclude everything from Docker build context that is not used by COPY steps in the Dockerfile.

This speeds up the container build on macOS, Windows or remote builders as less files and directories are copied over to the container builder. 

https://docs.docker.com/build/concepts/context/#dockerignore-files

.dockerignore

@@ -0,0 +1,4 @@
+# Exclude everything from context that is not used by COPY steps in the Dockerfile
+*
+!/target/s3proxy
+!/src/main/resources/run-docker-container.sh