chore: update alpine base to v3.15.4 (#2202)

docker-base/Dockerfile

@@ -1,7 +1,7 @@
 # This Dockerfile builds our base image with gosu, dumb-init and the atlantis
 # user. We split this from the main Dockerfile because this base doesn't change
 # and also because it kept breaking the build due to flakiness.
-FROM alpine:3.15.2
+FROM alpine:3.15.4
 LABEL authors="Anubhav Mishra, Luke Kysow"
 
 # We use gosu to step down from root and run as the atlantis user so we need