Bump files to use alpine:3.8

Bump Dockerfiles and mentions of alpine 3.7 to be now
3.8

Signed-off-by: Martin Dekov (VMware) <mdekov@vmware.com>

sample-functions/ResizeImageMagick/Dockerfile.armhf

-FROM arm32v6/alpine:3.7
+FROM arm32v6/alpine:3.8
 
 RUN apk --no-cache add imagemagick
 

sample-functions/WebhookStash/Dockerfile

@@ -14,7 +14,7 @@ COPY handler.go .
 
 RUN go install
 
-FROM alpine:3.7
+FROM alpine:3.8
 
 # Needed to reach the hub
 RUN apk --no-cache add ca-certificates 

sample-functions/figlet/Dockerfile.armhf

-FROM alpine:3.7
+FROM alpine:3.8
 
 RUN apk --no-cache add curl \
     && curl -sL https://github.com/openfaas/faas/releases/download/0.9.6/fwatchdog-armhf > /usr/bin/fwatchdog \

sample-functions/gif-maker/Dockerfile

-FROM alpine:3.7
+FROM alpine:3.8
 
 RUN apk --no-cache add ffmpeg gifsicle curl \
-   && curl -LS https://github.com/openfaas/faas/releases/download/0.9.6/fwatchdog > /usr/bin/fwatchdog \
-   && chmod +x /usr/bin/fwatchdog \
-   && apk del curl
+    && curl -LS https://github.com/openfaas/faas/releases/download/0.9.6/fwatchdog > /usr/bin/fwatchdog \
+    && chmod +x /usr/bin/fwatchdog \
+    && apk del curl
 WORKDIR /root/
 COPY entry.sh   .
 ENV fprocess="./entry.sh"