Pin to Alpine 3.11 so upgrade doesnt try to delete /var/run 📌

We're not quite at the bottom of
#2738 but it seems like
alpine 3.12 is having this problem and 3.11 is not. 3.12 seems to be
still being updated which may be why we are seeing different behavior
across runs using 3.12; for now pinning to 3.11 should be a way to be
able to build our releases while we try to understand what's going on.

images/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.11
 
 RUN apk add --update git openssh-client \
     && apk update \