Dachs discovers the name of linked Postgres host/container

Before, Postgres container should be named 'postgres'
to make the connection work, now Dachs figures out the
name of it from container's environment variable.
Needless to say the --link (legacy link) is still
mandatory.
TODO: move legacy-link to use (virtual) network.

dockerfile/bin/dachs.sh

@@ -2,30 +2,34 @@
 
 echo ''
 echo '================================================'
-echo 'This the (main) DaCHS server container, where'
-echo 'DaCHS daemon is meant to run.'
-echo 'This container expects to use another container'
-echo 'as the PostgreSQL databse server.'
+echo 'This is the DaCHS server container,'
+echo 'where DaCHS daemon runs.'
 echo ''
-echo 'The Postgres container must be named *postgres*.'
-echo 'Using the proper name will make the connection'
-echo '"just work".'
+echo 'This container expects to use another container'
+echo 'as the PostgreSQL database server.'
+echo 'This "expectation" is satisfied with Docker'
+echo 'option --link <name-of-postgres-container>.'
 echo ''
-echo 'Apart from that restriction, DaCHS management'
-echo 'proceed as usual (imp, pub, etc.).'
+echo 'If something doesnt work as expected, issue-us:'
+echo '- https://github.com/chbrandt/docker-dachs'
 echo '================================================'
 echo ''
 
+# get the name of the postgres container linked to this one
+PG_ENV_ALIAS=$(env | grep "ENV_PG_VERSION" | cut -d"_" -f1)
+PG_HOST=$(basename `env | grep "${PG_ENV_ALIAS}_NAME" | cut -d"=" -f2`)
+export PG_HOST
+
 # first, make sure the environment is initialised (can't do that
 # at image build time since the postgres container is not available then)
 echo -n "Waiting for postgres to come up..."
-while ! su - dachsroot -c "psql -h postgres --quiet gavo -c 'SELECT 1' > /dev/null 2>&1"  ;
+while ! su - dachsroot -c "psql -h ${PG_HOST} --quiet gavo -c 'SELECT 1' > /dev/null 2>&1"  ;
 do
     sleep 5
     echo -n .
 done
 echo
 
-su dachsroot -c "gavo init -d 'host=postgres dbname=gavo'"
+su dachsroot -c "gavo init -d 'host=${PG_HOST} dbname=gavo'"
 
 gavo serve start