build: add project dockerfile (#2)

Add a project Dockerfile to allow deploying the application in a docker
container. Add a `run.sh` script to start the application in the container.
Add Makefile commands to build the docker image based on the Dockerfile.
Fix possible nil reference error in the `db` package.

docker/run.sh

@@ -0,0 +1,9 @@
+#/bin/sh
+
+# Print version to logs for debugging purposes
+/app/fail2ban-prometheus-exporter -version
+
+# Start the exporter (use exec to support graceful shutdown)
+# Inspired by: https://akomljen.com/stopping-docker-containers-gracefully/
+exec /app/fail2ban-prometheus-exporter \
+    -db /app/fail2ban.sqlite3