Update Alpine version of Docker image

Update the Alpine version used by the Docker container that Jenkins
uses. This updates Node from v12 to v14 amongst other changes.

Dockerfile

@@ -3,7 +3,7 @@
 # upgrade everything by simply updating the Alpine version.
 #
 # Alpine is updated every 6 months so all packages are pretty recent.
-FROM alpine:3.12
+FROM alpine:3.13
 
 RUN apk update && apk add --no-cache \
   chromium \