CI: Stop building OCI images on each PR, to save resources

The test suite itself is already pretty heavy. It is not much of a
problem, because GHA offers to build OCI images on demand interactively,
in order to be able to still ship images for testing, if we need it.

.github/workflows/oci-runtime.yml

@@ -4,11 +4,20 @@
 name: OCI for runtime
 
 on:
-  pull_request: ~
+
+  # Build images when merging to `main` and when running a release.
   push:
+    branches:
+      - 'main'
     tags:
       - '*.*.*'
 
+  # Build images on each pull request.
+  # Remark: Activate only when needed, it will eat a lot of resources. The alternative
+  #         is to build manually / on-demand, by using the `workflow_dispatch` feature,
+  #         available through the GHA web UI.
+  # pull_request: ~
+
   # Produce a nightly image every day at 6 a.m. CEST.
   schedule:
     - cron: '0 4 * * *'
@@ -45,10 +54,11 @@ jobs:
             ghcr.io/${{ env.IMAGE_NAME }}
           # Generate OCI image tags based on the following events/attributes
           tags: |
-            type=schedule,pattern=nightly
+            type=ref,event=branch
             type=ref,event=pr
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
+            type=schedule,pattern=nightly
 
       - name: Inspect metadata
         run: |

.github/workflows/oci-server.yml

@@ -4,11 +4,20 @@
 name: OCI for server
 
 on:
-  pull_request: ~
+
+  # Build images when merging to `main` and when running a release.
   push:
+    branches:
+      - 'main'
     tags:
       - '*.*.*'
 
+  # Build images on each pull request.
+  # Remark: Activate only when needed, it will eat a lot of resources. The alternative
+  #         is to build manually / on-demand, by using the `workflow_dispatch` feature,
+  #         available through the GHA web UI.
+  # pull_request: ~
+
   # Produce a nightly image every day at 6 a.m. CEST.
   schedule:
     - cron: '0 4 * * *'
@@ -83,10 +92,11 @@ jobs:
             ghcr.io/${{ env.IMAGE_NAME }}
           # Generate OCI image tags based on the following events/attributes
           tags: |
-            type=schedule,pattern=nightly
+            type=ref,event=branch
             type=ref,event=pr
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
+            type=schedule,pattern=nightly
 
       - name: Inspect metadata
         run: |

docs/backlog.md

@@ -23,9 +23,6 @@
 
 ## Iteration +4
 - UX: CLI shortcut for `ddl/drop.sql`
-- CI: Do not build OCI images on _each_ PR in the long run, it costs too many
-  resources. Instead, document how to run OCI builds on demand on specific
-  branches, when it is needed to ship images for testing purposes.
 
 
 ## Done
@@ -34,3 +31,6 @@
 - Add example `tracking_merlion.py`
 - Integrate tutorial from `cratedb-examples`
 - Add GHA recipe to build and ship OCI image
+- CI: Do not build OCI images on _each_ PR in the long run, it costs too many
+  resources. Instead, document how to run OCI builds on demand on specific
+  branches, when it is needed to ship images for testing purposes.