Also enables support for Alpine post-upgrade script with warning
if it's necessary to run update-conf to apply new version of files
in /etc:
```
...
Executing one-context-6.1.80-r0.20210809gitbcd3c5b.post-upgrade
WARNING: Run update-conf to process any updated one-context files in /etc!
Executing busybox-1.33.1-r2.trigger
Executing eudev-3.2.10-r0.trigger
OK: 148 MiB in 118 packages
$ update-conf -l
//etc/one-context.d/loc-09-timezone
//etc/one-context.d/loc-30-console
//etc/one-context.d/loc-10-network
//etc/one-context.d/loc-16-gen-env
```
Closes#163
Vlastimil Holer
Petr Ospalý
Vlastimil Holer
Vlastimil Holer