Update Dockerfile - alpine:3.12 -> alpine:3.15

The support for the base image alpine:3.12 will expire in 4 months (https://endoflife.date/alpine), so it would make sense to upgrade to the current version alpine:3.15. 
I was able to create the acme.sh image with the new alpine:3.15 version without errors and also create and deploy a certificate, but further testing would be useful.

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.15
 
 RUN apk --no-cache add -f \
   openssl \