Update alpine Docker tag to v3.15 (#170)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

packaging/docker/x86-alpine/Dockerfile

@@ -1,14 +1,14 @@
 # use multi stage dockerfile to first create a virtualenv,
 # then copy the virtualenv (without all build deps) into target image
 
-FROM alpine:3.14 as build
+FROM alpine:3.15 as build
 RUN apk -U update && apk -U upgrade
 RUN apk -U add python3 py3-virtualenv python3-dev gcc musl-dev linux-headers
 ADD src /src
 RUN virtualenv /usr/local/dyndnsc && \
     /usr/local/dyndnsc/bin/pip install /src
 
-FROM alpine:3.14
+FROM alpine:3.15
 RUN apk -U update && \
     apk -U upgrade && \
     apk -U add --no-cache python3 && \