This repository provides easy to download Oracle instantclient used in Dockerfiles. Thank you. Take a look at installation guide for Ubuntu. Here is example Dockerfile, where you can find PHP 7.2 with OCI8 extension and Oracle instantclient v12.2.0.1.0.
Oracle Instant Client Downloads for Linux x86-64 (64-bit) See the Instant Client Home Page for more information about Instant Client. The installation instructions are at the foot of the page. Client-server version interoperability is detailed in Doc ID 2
Instant Client downloads for z/Linux (31-bit and 64-bit) See the Instant Client Home Page for more information about Instant Client. The installation instructions are at the foot of the page.