Move docker files in docker dir and add instructions

docker/Makefile

@@ -0,0 +1,13 @@
+REGISTRY=docker.io
+IMAGE=terracotta
+TAG=latest
+
+all: build push
+
+build:
+	docker build -f Dockerfile -t $(REGISTRY)/$(IMAGE):$(TAG) ..
+
+push:
+	docker push $(REGISTRY)/$(IMAGE):$(TAG)
+
+.PHONY: build push all

docker/README.md

@@ -0,0 +1,29 @@
+# Terracotta docker
+
+This directory contains the docker files to build the Terracotta docker image.
+
+## Build the image
+**Make sure the current directory is in docker/**
+
+Build using the default image name and tag:
+```bash
+make build
+```
+
+Build using a custom registry, image name and tag:
+```bash
+make build REGISTRY=myregistry.com IMAGE=terracotta TAG=test
+```
+
+## Push the image
+**Make sure the current directory is in docker/**
+
+Push using the default image name and tag:
+```bash
+make push
+```
+
+Push using a custom registry, image name and tag:
+```bash
+make push REGISTRY=myregistry.com IMAGE=terracotta TAG=test
+```