fix redis version in server docker image

cmd/server/Dockerfile

@@ -54,7 +54,7 @@ RUN apk update && apk add --no-cache \
  # the features we can depend on. See this link for more information:
  # https://github.com/sourcegraph/sourcegraph/blob/master/doc/dev/postgresql.md#version-requirements
  'bash=5.0.0-r0' 'postgresql-contrib=11.7-r0' 'postgresql=11.7-r0' \
- 'redis=5.0.8-r0' bind-tools ca-certificates git@edge \
+ 'redis=5.0.5-r0' bind-tools ca-certificates git@edge \
  mailcap nginx openssh-client pcre su-exec tini nodejs-current=12.4.0-r0 curl
 
 # IMPORTANT: If you update the syntect_server version below, you MUST confirm