Merge the Complement testing Docker images into a single, multi-purpose image. (#12881)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

1 files changed, 32 insertions, 1 deletions

diff --git a/docker/complement/README.md b/docker/complement/README.md
index e075418e4a..37c39e2dfc 100644
--- a/docker/complement/README.md
+++ b/docker/complement/README.md
@@ -1 +1,32 @@
-Stuff for building the docker image used for testing under complement.
+# Unified Complement image for Synapse
+
+This is an image for testing Synapse with [the *Complement* integration test suite][complement].
+It contains some insecure defaults that are only suitable for testing purposes,
+so **please don't use this image for a production server**.
+
+This multi-purpose image is built on top of `Dockerfile-workers` in the parent directory
+and can be switched using environment variables between the following configurations:
+
+- Monolithic Synapse with SQLite (`SYNAPSE_COMPLEMENT_DATABASE=sqlite`)
+- Monolithic Synapse with Postgres (`SYNAPSE_COMPLEMENT_DATABASE=postgres`)
+- Workerised Synapse with Postgres (`SYNAPSE_COMPLEMENT_DATABASE=postgres` and `SYNAPSE_COMPLEMENT_USE_WORKERS=true`)
+
+The image is self-contained; it contains an integrated Postgres, Redis and Nginx.
+
+
+## How to get Complement to pass the environment variables through
+
+To pass these environment variables, use [Complement's `COMPLEMENT_SHARE_ENV_PREFIX`][complementEnv]
+variable to configure an environment prefix to pass through, then prefix the above options
+with that prefix.
+
+Example:
+```
+COMPLEMENT_SHARE_ENV_PREFIX=PASS_ PASS_SYNAPSE_COMPLEMENT_DATABASE=postgres
+```
+
+Consult `scripts-dev/complement.sh` in the repository root for a real example.
+
+
+[complement]: https://github.com/matrix-org/complement
+[complementEnv]: https://github.com/matrix-org/complement/pull/382