build: add healthcheck to docker image (!109)

* Update docker image to include a healthcheck instruction using a custom script
* Replace docker base image with alpine

https://gitlab.com/hectorjsmith/fail2ban-prometheus-exporter/-/merge_requests/109

Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:buster-slim
+FROM alpine
 
 # Create main app folder to run from
 WORKDIR /app
@@ -6,5 +6,10 @@ WORKDIR /app
 # Copy compiled binary to release image
 # (must build the binary before running docker build)
 COPY fail2ban_exporter /app/fail2ban_exporter
+COPY health /app/health
+
+# Setup a healthcheck
+RUN apk add curl
+HEALTHCHECK --interval=10s --timeout=4s CMD /app/health
 
 ENTRYPOINT ["/app/fail2ban_exporter"]

health

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+port=9191
+if [ ! -z $F2B_WEB_LISTEN_ADDRESS ]; then
+    port=`echo $F2B_WEB_LISTEN_ADDRESS | cut -d ":" -f 2 -`
+fi
+
+curl --fail localhost:$port/metrics || exit 1