Updates the installer library to be static in the final image, saving the installers into Git and curl-ing the correct revision

docker-builders/README.md

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+# Installer Library
+
+This folder contains the Dockerfiles for building certain installers or libraries, which are then pulled into the main image.
+
+## [jbig2enc](https://github.com/agl/jbig2enc)
+
+### Why
+
+JBIG is an image coding which can achieve better compression of images for PDFs.
+
+### What
+
+The Docker image builds a shared library file and utility, which is copied into the correct location in the final image.
+
+See Also:
+
+- [OCRMyPDF Documentation](https://ocrmypdf.readthedocs.io/en/latest/jbig2.html)
+
+## [psycopg2](https://www.psycopg.org/)
+
+### Why
+
+The pre-built wheels of psycopg2 are built on Debian 9, which provides a quite old version of libpq-dev. This causes issue with authentication methods.
+
+### What
+
+The image builds psycopg2 wheels on Debian 10 and places the produced wheels into `/usr/src/wheels/`.
+
+See Also:
+
+- [Issue 266](https://github.com/paperless-ngx/paperless-ngx/issues/266)
+
+## [qpdf](https://qpdf.readthedocs.io/en/stable/index.html)
+
+### Why
+
+qpdf and it's library provide tools to read, manipulate and fix up PDFs. Version 11 is also required by `pikepdf` 6+ and Debian 9 does not provide above version 10.
+
+### What
+
+The Docker image cross compiles .deb installers for each supported architecture of the main image. The installers are placed in `/usr/src/qpdf/${QPDF_VERSION}/${TARGETARCH}${TARGETVARIANT}/`
+
+## [pikepdf](https://pikepdf.readthedocs.io/en/latest/)
+
+### Why
+
+Required by OCRMyPdf, this is a general purpose library for PDF manipulation in Python via the qpdf libraries.
+
+### What
+
+The built wheels are placed into `/usr/src/wheels/`