Update Alpine in CI to v3.12

Update the software used to run tests in CI. This _may_ help with
#2249.

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:3.10
+FROM alpine:3.12
 
 RUN apk update && apk add --no-cache \
   chromium \