This is a basic example of extending the postgis/postgis image to make CLIs available (#224) (#229)

* Added basic example

* Addressed comment on pull request #229

Author: zachsa

examples/image-with-postgis-clis/Dockerfile

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+FROM postgis/postgis:13-3.1
+
+RUN apt-get update -y
+RUN apt-get install postgis -y
+RUN apt-get clean
+RUN rm -rf /var/cache/apt/lists

examples/image-with-postgis-clis/README.md

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+# Using PostGIS CLIs
+The base postgis/postgis image does not have PostGIS-related CLIs installed. To use PostGIS CLIs that are NOT installed by default (for example `raster2pgsql`) it's necessary to extend the base image.
+
+```sh
+# Create a Docker image
+docker build -t my-postgis .
+
+# Run as a Docker container
+docker run --name my-postgis -p 5432:5432 -e POSTGRES_USER=admin -e POSTGRES_PASSWORD=password -d my-postgis
+```