deps: alpine 3.15 (#1941)

Signed-off-by: Rui Chen <[email protected]>

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:3.14
+FROM alpine:3.15
 LABEL authors="Anubhav Mishra, Luke Kysow"
 
 # We use gosu to step down from root and run as the atlantis user so we need