Dockerfile: use alpine 3.12

In Alpine 3.11 and 3.13, 'nslookup' exits with the error code 1 if it
can't resolve all IPs for the search list defined in /etc/resolv.conf

However, it seems that Alpine 3.10 and 3.12 are not affected by this bug
and continue return the error code 0 if at least on of the domains in
the search list is resolved into an IP address. Thus, we will use the
latest Alpine image available for the 3.12 release series.

Signed-off-by: André Martins <andre@cilium.io>

contrib/coccinelle/Dockerfile

@@ -1,4 +1,6 @@
-FROM docker.io/library/alpine:3.13.1@sha256:08d6ca16c60fe7490c03d10dc339d9fd8ea67c6466dea8d558526b1330a85930
+# Do not upgrade to alpine 3.13 as its nslookup tool returns 1, instead of 0
+# for domain name lookups.
+FROM docker.io/library/alpine:3.12.7@sha256:36553b10a4947067b9fbb7d532951066293a68eae893beba1d9235f7d11a20ad
 
 LABEL maintainer="maintainer@cilium.io"
 

examples/kubernetes-memcached/memcd-sw-app.yaml

@@ -71,7 +71,7 @@ spec:
     spec:
       containers:
       - name: starfigher
-        image: alpine
+        image: docker.io/library/alpine:3.12.7@sha256:36553b10a4947067b9fbb7d532951066293a68eae893beba1d9235f7d11a20ad
         command: ["sleep"]
         args: ["30000"]
 ---