docs: Extend build guidance with secrets

It's really worth emphasizing the authenticated-registry flow.

Author: cgwalters

docs/src/SUMMARY.md

@@ -10,6 +10,7 @@
 
 - [Building images](building/guidance.md)
 - [Users, groups, SSH keys](building/users-and-groups.md)
+- [Secrets](building/secrets.md)
 
 # Using bootc
 

docs/src/building/guidance.md

@@ -50,8 +50,8 @@ and if you embed software in your derived image, the
 default would then be that that software is initially
 launched via a systemd unit.
 
-```
-RUN dnf -y install postgresql
+```dockerfile
+RUN dnf -y install postgresql && dnf clean all
 ```
 
 Would typically also carry a systemd unit, and that
@@ -88,3 +88,7 @@ make in the container image to e.g. `/etc/postgresql.conf`
 will be applied on update, assuming it is not modified
 locally.
 
+### Secrets
+
+There is a dedicated document for [secrets](secrets.md),
+which is a special case of configuration.

docs/src/building/secrets.md

@@ -0,0 +1,51 @@
+
+# Secrets (e.g. container pull secrets)
+
+To have `bootc` fetch updates from registry which requires authentication,
+you must include a pull secret in `/etc/ostree/auth.json`.
+
+Another common case is to also fetch container images via
+`podman` or equivalent.  There is a [pull request to add `/etc/containers/auth.json`](https://github.com/containers/image/pull/1746)
+which would be shared by the two stacks by default.
+
+Regardless, injecting this data is a good example of a generic
+"secret".  The bootc project does not currently include one
+single opinionated mechanism for secrets.
+
+## Embedding in container build
+
+This was mentioned above; you can include secrets in
+the container image if the registry server is suitably protected.
+
+In some cases, embedding only "bootstrap" secrets into the container
+image is a viable pattern, especially alongside a mechanism for
+having a machine authenticate to a cluster.   In this pattern,
+a provisioning tool (whether run as part of the host system
+or a container image) uses the bootstrap secret to lay down
+and keep updated other secrets (for example, SSH keys,
+certificates).
+
+## Via cloud metadata
+
+Most production IaaS systems support a "metadata server" or equivalent
+which can securely host secrets - particularly "bootstrap secrets".
+Your container image can include tooling such as `cloud-init`
+or `ignition` which fetches these secrets.
+
+## Embedded in disk images
+
+Another pattern is to embed bootstrap secrets only in disk images.
+For example, when generating a cloud disk image (AMI, OpenStack glance image, etc.)
+from an input container image, the disk image can contain secrets that
+are effectively machine-local state.  Rotating them would
+require an additional management tool, or refreshing disk images.
+
+## Injected via baremetal installers
+
+It is common for installer tools to support injecting configuration
+which can commonly cover secrets like this.
+
+## Injecting secrets via systemd credentials
+
+The systemd project has documentation for [credentials](https://systemd.io/CREDENTIALS/)
+which applies in some deployment methodologies.