Update README with instructions for new containers

README.md

@@ -1 +1,47 @@
-# pressio-containers
+# pressio-containers
+
+This repository holds all of the Dockerfiles that create the containers used throughout the Pressio ecosystem for testing.
+These containers allow us to, e.g., not rebuild Trilinos every time we test Pressio in CI.
+
+When a Dockerfile is modified or added to this repository, the `ci-docker.yml` GitHub workflow will automatically build and push the image to the [Pressio container registry](https://github.com/orgs/Pressio/packages).
+
+## Adding a New Container
+
+Generally, to add a new container you will need to create a new Dockerfile (see the `docker_scripts` directory for existing Dockerfiles).
+
+Once the dockerfile has been made, simply add it to the configuration matrix of the `ci-docker.yml`.
+
+For example, if the new file is called `macos-clang.dockerfile`, you would add to the matrix:
+
+```yaml
+jobs:
+  CI:
+    strategy:
+      matrix:
+        config:
+          #
+          # Current containers...
+          #
+          - {dockerfile: 'macos-clang', compiler-version: '14', tag: 'latest'}
+```
+
+## Adding a New Version of Trilinos
+
+To add a new version of Trilinos,  you **do not** need to create a new Dockerfile.
+Instead, use the existing `ubuntu-gnu-trilinos.dockerfile` file in the configuration matrix of `ci-docker.yml` and add the desired tag or commit hash to the `tag` field.
+
+For example, to add an image with Trilinos 15.0.0 (tagged in GH with `trilinos-release-15-0-0`):
+
+```yaml
+jobs:
+  CI:
+    strategy:
+      matrix:
+        config:
+          #
+          # Current containers...
+          #
+          - {dockerfile: 'ubuntu-gnu-trilinos', compiler-version: '11', tag: 'trilinos-release-15-0-0'}
+```
+
+See how Trilinos `702aac5` was added in https://github.com/Pressio/pressio/pull/651.