Switched to using Alpine latest

Dockerfile

@@ -1,11 +1,10 @@
-FROM ubuntu:18.04
-RUN apt-get update && apt-get upgrade -y && apt-get install wget gnupg expect -y && \
-    wget -O - https://packages.icinga.com/icinga.key | apt-key add -
-RUN printf "deb http://packages.icinga.com/ubuntu icinga-bionic main\ndeb-src http://packages.icinga.com/ubuntu icinga-bionic main" > /etc/apt/sources.list.d/icinga2.list && \
-    apt-get update && apt-get install icinga2 -y && \
-    mkdir /run/icinga2 && chown nagios:nagios /run/icinga2 && \
+FROM alpine:latest
+RUN apk update && \
+    apk add bash && \
+    apk add icinga2 && \
+    mkdir /run/icinga2 && chown icinga:icinga /run/icinga2 && \
     mkdir -p /var/lib/icinga2/certs && \
-    chown -R nagios:nagios /var/lib/icinga2/certs
+    chown -R icinga:icinga /var/lib/icinga2/certs
 
 COPY create-satellite.sh /create-satellite.sh
 COPY run-icinga.sh /run-icinga.sh

README.md

@@ -53,3 +53,11 @@ services:
       - DISABLE_CONFD=y
 ```
 
+## Images
+There are two available images for you to choose from. The default one (0.*n*) is based on
+Debian 10, with Icinga2 from Icingas official repository. The other image (0.*n*-alpine) is
+based on Alpine latest, with Icinga2 from Alpines repository. The Alpine image is much
+smaller in size.
+
+> **Note:** Version 0.1-alpine uses Alpine 3.11. Version 0.1.1-alpine and up uses Alpine
+> latest.