libpq is missing from docker images based on alpine 3.9

[imb590]

require('pg') causes the following error:

LuajitError: error: "/.rocks/share/tarantool/pg/init.lua:4: module 'pg.driver' not found:
	no field package.preload['pg.driver']
	no file '/opt/tarantool/pg/driver.lua'
	no file '/opt/tarantool/pg/driver/init.lua'
	no file '/opt/tarantool/pg/driver.so'
	no file '/opt/tarantool/.rocks/share/tarantool/pg/driver.lua'
	no file '/opt/tarantool/.rocks/share/tarantool/pg/driver/init.lua'
	no file '/opt/.rocks/share/tarantool/pg/driver.lua'
	no file '/opt/.rocks/share/tarantool/pg/driver/init.lua'
	no file '/.rocks/share/tarantool/pg/driver.lua'
	no file '/.rocks/share/tarantool/pg/driver/init.lua'
	Error loading shared library libpq.so.5: No such file or directory (needed by /.rocks/lib/tarantool/pg/driver.so)
	no file '/home/tarantool/.luarocks/share/lua/5.1/pg/driver.lua'
	no file '/home/tarantool/.luarocks/share/lua/5.1/pg/driver/init.lua'
	no file '/home/tarantool/.luarocks/share/lua/pg/driver.lua'
	no file '/home/tarantool/.luarocks/share/lua/pg/driver/init.lua'
	no file '/usr/local/share/tarantool/pg/driver.lua'
	no file '/usr/local/share/tarantool/pg/driver/init.lua'
	no file '/usr/share/tarantool/pg/driver.lua'
	no file '/usr/share/tarantool/pg/driver/init.lua'
	no file '/usr/local/share/lua/5.1/pg/driver.lua'
	no file '/usr/local/share/lua/5.1/pg/driver/init.lua'
	no file '/usr/share/lua/5.1/pg/driver.lua'
	no file '/usr/share/lua/5.1/pg/driver/init.lua'
	no file '/home/tarantool/.luarocks/lib/lua/5.1/pg/driver.so'
	no file '/home/tarantool/.luarocks/lib/lua/pg/driver.so'
	no file '/usr/local/lib64/tarantool/pg/driver.so'
	no file '/usr/local/lib/tarantool/pg/driver.so'
	no file '/usr/lib64/tarantool/pg/driver.so'
	no file '/usr/local/lib64/lua/5.1/pg/driver.so'
	no file '/usr/local/lib/lua/5.1/pg/driver.so'
	no file '/usr/lib64/lua/5.1/pg/driver.so'
	no file '/home/tarantool/.luarocks/lib/lua/5.1/pg.so'
	no file '/home/tarantool/.luarocks/lib/lua/pg.so'
	no file '/usr/local/lib64/tarantool/pg.so'
	no file '/usr/local/lib/tarantool/pg.so'
	no file '/usr/lib64/tarantool/pg.so'
	no file '/usr/local/lib64/lua/5.1/pg.so'
	no file '/usr/local/lib/lua/5.1/pg.so'
	no file '/usr/lib64/lua/5.1/pg.so'"

because the dependency of /.rocks/lib/tarantool/pg/driver.so is not present.

It seems that in alpine 3.9 apk add treats the second call with the same virtual package --virtual run-deps here as no-op and does not install anything.