Bump alpine image for security fix

This change includes the fix at
https://github.com/alpinelinux/apk-tools/commit/6484ed9849f03971eb48ee1fdc21a2f128247eb1

Signed-off-by: Nolan Brubaker <nolan@heptio.com>

Dockerfile-ark-restic-restore-helper.alpine

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM alpine:3.7
+FROM alpine:3.8
 
 MAINTAINER Steve Kriss <steve@heptio.com>
 
@@ -20,4 +20,4 @@ ADD /bin/linux/amd64/ark-restic-restore-helper .
 
 USER nobody:nobody
 
-ENTRYPOINT [ "/ark-restic-restore-helper" ]
\ No newline at end of file
+ENTRYPOINT [ "/ark-restic-restore-helper" ]

Dockerfile-ark.alpine

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM alpine:3.7
+FROM alpine:3.8
 
 MAINTAINER Andy Goldstein <andy@heptio.com>
 

hack/build-image/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM golang:1.10-alpine3.7
+FROM golang:1.10-alpine3.8
 
 RUN apk add --update --no-cache git bash && \
     mkdir -p /go/src/k8s.io && \