Document how to update python requirements file
Documenting these steps will allow future updates to be more consistent.
This is a follow-up to [1].
[1] https://gerrit-review.googlesource.com/c/aws-gerrit/+/389014/comment/add73286_b60daad9/
Bug: Issue 307592551
Change-Id: I3a6f302eb5b2a166de7cb408507ca2dc9471f10e
diff --git a/gerrit/README.md b/gerrit/README.md
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+## Gerrit Docker image
+
+This project defines the Docker image to be used with the AWS recipes, as described in the main
+[README](../README.md).
+
+### Generate Python requirements file
+
+Occasionally, we need to update the Python dependencies. This could be due to
+upgrading the base image, or we may just want to upgrade the libraries.
+
+Follow these steps to generate a new requirements file for reproducible builds.
+
+```bash
+# This needs to match the base image of the Gerrit build
+docker run --rm -it almalinux:9 bash
+
+yum install python3 python3-libs python3-devel python3-pip
+
+cd
+python3 -m venv venv
+
+source venv/bin/activate
+
+# These are our direct dependencies
+pip install boto3==1.23.10 jinja2==2.11.1 awscli==1.24.10
+
+# Required for compatibility with the pinned version of jinja2
+pip install markupsafe==2.0.1
+
+pip freeze > requirements.in
+
+python3 -m pip install pip-tools
+pip-compile --generate-hashes requirements.in
+```
