Es ist in Ordnung! Mir fehlte eine .conf in /etc/ld.so.conf.d/oracle-instantclient.conf
RUN echo "instantclient,/usr/lib/oracle/12.2/instantclient"| pecl install oci8 && \
echo "extension=oci8.so" > /etc/php/7.0/mods-available/oci8.ini && \
ln -s /etc/php/7.0/mods-available/oci8.ini /etc/php/7.0/fpm/conf.d/20-oci8.ini && \
echo /usr/lib/oracle/12.2/instantclient > /etc/ld.so.conf.d/oracle-instantclient.conf && \
ldconfig
