Merge pull request #52 from lsst-dm/u/ktl/document-build

Add build instructions.

README.md

@@ -4,3 +4,19 @@ embargo-butler
 This package provides the services needed to receive notifications of
 delivery of images from the Camera Control System to an object store bucket
 and automatically ingest them into an appropriate Butler repository.
+
+Containers
+----------
+
+Three containers are built from this repo: enqueue, ingest, and idle.
+
+Building
+--------
+
+Pull requests will build versions of all three containers tagged with the PR's "head ref".
+If a tag (which should always be on main and prefixed with "v") is pushed, versions of all three containers with that tag's version number will be built.
+Otherwise, merges to main will result in versions of all three containers tagged with "latest".
+
+If the code in this repo has not changed but a new version of the Science Pipelines stack is needed for the ingest container, the ["On-demand ingest build" workflow](https://github.com/lsst-dm/embargo-butler/actions/workflows/build-manually.yaml) should be executed.
+It takes a ref — which should be the latest tag, not usually a branch — a rubin-env version, and the Science Pipelines release tag (e.g. `w_2023_41`).
+The tag for the resulting ingest container will contain both the version from the tag of this repo plus the Science Pipelines release tag (e.g. `0.99.2-w_2023_41`).