Here is a working solution for the official PHP-FPM images based on Debian 10 (Buster). The following Dockerfile installs the Oracle Instant Client 18.5 (basiclite and devel) using the RPM packges and alien.
As Christopher Jones wrote the files can currently be downloaded without an Oracle account.
FROM php:7.2.32-fpm
# see https://help.ubuntu.com/community/Oracle%20Instant%20Client
RUN apt-get update && apt-get install -y --no-install-recommends alien libaio1 wget && \
wget https://download.oracle.com/otn_software/linux/instantclient/185000/oracle-instantclient18.5-basiclite-18.5.0.0.0-3.x86_64.rpm && \
wget https://download.oracle.com/otn_software/linux/instantclient/185000/oracle-instantclient18.5-devel-18.5.0.0.0-3.x86_64.rpm && \
alien -i oracle-instantclient18.5-basiclite-18.5.0.0.0-3.x86_64.rpm && \
alien -i oracle-instantclient18.5-devel-18.5.0.0.0-3.x86_64.rpm
ENV LD_LIBRARY_PATH="/usr/lib/oracle/18.5/client64/lib:${LD_LIBRARY_PATH}"