README: add build particularities

README.md

@@ -1,3 +1,13 @@
 # chromium-swiftshader-alpine
 
 An alpine-based container image with `chromium-swiftshader` installed. This is released and tagged with the `chromium-swiftshader` version inside, which allow downstream packages that need chromium to pin this version safely.
+
+## Using this image
+
+This image is tagged as `ghcr.io/grafana/chromium-swiftshader-alpine:CHROMIUMVERSION-ALPINEVERSION`, with both versions grepped from the `Dockerfile`. As the image is tagged on every push to `main`, it is unlikely but possible for that tag to be overwritten if changes to CI/CD or similar are merged. It is recommended to pin the digest of it:
+
+```Dockerfile
+FROM ghcr.io/grafana/chromium-swiftshader-alpine:000.0.0000.00-r0-0.0.0@sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
+```
+
+That way, dependency bumping bots will create PRs when the tag is overwritten.