chore: Update aptly to 1.6.2

Update aptly to 1.6.1

to fix segfault #23

Update dockerpublish.yml

Dockerfile: Update aptly to 1.5.0

https://github.com/aptly-dev/aptly/releases/tag/v1.5.0

Signed-off-by: Remus Wu <remuswu1019@gmail.com>

Update keys_gen.sh

Directories must be `0700` to be able to be listed / have files created in the,

assets: update_mirror.sh: Usage prints help about environment variables

Users can find the information more easily than comments in the script.

Signed-off-by: Kazuhiro Hayashi <kazuhiro3.hayashi@toshiba.co.jp>

Change keyserver from SKS to Ubuntu

The SKS keyserver pool is deprecated, so for reliable builds another maintained keyserver should be used.
Ubuntu maintains another keyserver which seems supported, so use that instead.