add ability to use ahriman pacman database instead of system one (#71)

By default this feature is enabled. On the first run it will copy (if
exists) databases from filesystem to local cache (one per each
architecture). Later it will use this cache for all alpm operations. In
order to update this cache, some commands (mainly package building)
provide `-y`/`--refresh` option which has same semantics as pacman -Sy
does.

Note however that due to extending `Pacman` class some methods were
renamed in order to be more descriptive:
* `Pacman.all_packages` -> `Pacman.packages`
* `Pacman.get` -> `Pacman.package_get`

This commit also adds multilib repository to the default docker image
which was missed.

package/lib/systemd/system/ahriman@.service

@@ -2,6 +2,6 @@
 Description=ArcH linux ReposItory MANager (%I architecture)
 
 [Service]
-ExecStart=/usr/bin/ahriman --architecture %i update
+ExecStart=/usr/bin/ahriman --architecture %i repo-update --refresh
 User=ahriman
 Group=ahriman

package/share/ahriman/settings/ahriman.ini

@@ -5,8 +5,10 @@ database = /var/lib/ahriman/ahriman.db
 
 [alpm]
 database = /var/lib/pacman
+mirror = https://geo.mirror.pkgbuild.com/$repo/os/$arch
 repositories = core extra community multilib
 root = /
+use_ahriman_cache = yes
 
 [auth]
 target = disabled