docker: revert update alpine Docker tag to v3.14.0

- https://github.com/aquasecurity/trivy/releases/tag/v0.18.3 does not support Alpine 3.14 yet
- https://wiki.alpinelinux.org/wiki/Release_Notes_for_Alpine_3.14.0#faccessat2 poses problems on older Docker hosts, do not reduce compatibility right now

This reverts commit dcd4201523.

docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.14.0
+FROM alpine:3.13.5
 
 EXPOSE 5353 8080
 

docker/ci/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.14.0
+FROM alpine:3.13.5
 
 EXPOSE 5353 8080