Merge pull request #1136 from holzman/alpine-3.15

[MRG] Update Dockerfile to current Alpine (ALPINE_VERSION=3.15.0)

Dockerfile

@@ -1,4 +1,4 @@
-ARG ALPINE_VERSION=3.12.0
+ARG ALPINE_VERSION=3.15.0
 FROM alpine:${ALPINE_VERSION}
 
 RUN apk add --no-cache git python3 python3-dev py-pip build-base