Add a note on manually building images

[jarrah42]

In GitLab by @evilkermit on Apr 17, 2025, 11:43 EDT

https://nova.ornl.gov/tutorial/02-Copy-Template.html#continuous-integration-and-container-building doesn't make note of the difference between the tutorial (where build-image runs automatically) and real applications (where build-image runs manually).

Migrated from GitLab: https://code.ornl.gov/ndip/public-packages/nova-carpentry-tutorial/-/issues/33

[jarrah42]

In GitLab by @evilkermit on Apr 17, 2025, 13:19 EDT

Alternatively, we could make image builds happen automatically.