Add Dockerfile and custom image build instructions

Dockerfile

@@ -0,0 +1,2 @@
+FROM postgres:9.6
+COPY create-multiple-postgresql-databases.sh /docker-entrypoint-initdb.d/

README.md

@@ -15,6 +15,8 @@ mechanism.
 
 ## Usage
 
+### By mounting a volume
+
 Clone the repository, mount its directory as a volume into
 `/docker-entrypoint-initdb.d` and declare database names separated by commas in
 `POSTGRES_MULTIPLE_DATABASES` environment variable as follows
@@ -28,3 +30,21 @@ Clone the repository, mount its directory as a volume into
             - POSTGRES_MULTIPLE_DATABASES=db1,db2
             - POSTGRES_USER=myapp
             - POSTGRES_PASSWORD=
+
+### By building a custom image
+
+Clone the repository, build and push the image to your Docker repository,
+for example for Google Private Repository do the following:
+
+    docker build --tag=eu.gcr.io/your-project/postgres-multi-db
+    gcloud docker -- push eu.gcr.io/your-project/postgres-multi-db
+
+You still need to pass the `POSTGRES_MULTIPLE_DATABASES` environment variable
+to the container:
+
+    myapp-postgresql:
+        image: eu.gcr.io/your-project/postgres-multi-db
+        environment:
+            - POSTGRES_MULTIPLE_DATABASES=db1,db2
+            - POSTGRES_USER=myapp
+            - POSTGRES_PASSWORD=