Switched to using Alpine latest

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.11
+FROM alpine:latest
 RUN apk update && \
     apk add bash && \
     apk add icinga2 && \

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.