Pass the URL of the Oracle server with URL envvar:

$ docker run -e URL=<user>/<password>@//xxx.yyy.eu-west-1.rds.amazonaws.com:1521/ORCL -ti sflyr/sqlplus

SQL*Plus: Release 11.2.0.3.0 Production on Tue Aug 11 15:41:07 2015

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Conectado a:
Oracle Database 11g Release 11.2.0.1.0 - 64bit Production

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