allow inputing non-ascii char with psql

[allan-simon]

you can easily test this:

try without, then run

docker exec -it your_instance psql

versus

docker exec -it your_instance -c 'LD_PRELOAD=/lib/x86_64-linux-gnu/libreadline.so.6.3 TERM=xterm psql'

in one you can input non-ascii character (like french accent or cyrilics) , in the other you can't

[tianon]

See also #310 and thus #355 (which should've already fixed this):

$ docker pull postgres:9.6
9.6: Pulling from library/postgres
Digest: sha256:8df3344385deb0d5d781c442b2e275f7c321d601652d6317ce25ed8ffad03427
Status: Image is up to date for postgres:9.6

$ docker run -dit --name pg postgres:9.6
ea281fd1ca1d13b5482d761046e45d79755e160c4a272acc70d64c4caf3e37e2

$ docker exec -it pg psql -U postgres
psql (9.6.8)
Type "help" for help.

postgres=# SELECT 'נראה עובד?';
  ?column?  
------------
 נראה עובד?
(1 row)

postgres=# SELECT 'Кажется, работает?';
      ?column?      
--------------------
 Кажется, работает?
(1 row)