Fix bundle gems issue in docker

This is an issue with our docker alpine image and an older docker host. Keeping to apline3.13 fixes the below error
Fix for :
While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for/usr/local/bundle

Solution:
docker-library/ruby#351

Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:2.7.5-alpine3.15
+FROM ruby:2.7.5-alpine3.13
 LABEL Ministry of Justice, LAA Get Paid <laa-get-paid@digital.justice.gov.uk>
 
 # fail early and print all commands

docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:2.7.5-alpine
+FROM ruby:2.7.5-alpine3.13
 LABEL Organisation="Ministry of Justice"
 LABEL Team="LAA Get Paid"
 LABEL Contact="<laa-get-paid@digital.justice.gov.uk>"