Well, the lemmy container kept running into:

lemmy  | 2023-06-24T23:28:52.716586Z  INFO lemmy_server::code_migrations: Running apub_columns_2021_02_02
lemmy  | 2023-06-24T23:28:52.760510Z  INFO lemmy_server::code_migrations: Running instance_actor_2021_09_29
lemmy  | 2023-06-24T23:28:52.763723Z  INFO lemmy_server::code_migrations: Running regenerate_public_keys_2022_07_05
lemmy  | 2023-06-24T23:28:52.801409Z  INFO lemmy_server::code_migrations: Running initialize_local_site_2022_10_10
lemmy  | 2023-06-24T23:28:52.803303Z  INFO lemmy_server::code_migrations: No Local Site found, creating it.
lemmy  | thread 'main' panicked at 'couldnt create local user: DatabaseError(UniqueViolation, "duplicate key value violates unique constraint \"local_user_person_id_key\"")', crates/db_schema/src/impls/local_user.rs:157:8

despite the fact that:

lemmy=# select id, site_id from local_site;
 id | site_id
----+---------
  1 |       1
(1 row)

So you can see that it was unconditionally trying to create a local_site and running into a DB constraint error. I further narrowed it down to this piece of code:

///
/// If a site already exists, the DB migration should generate a local_site row.
/// This will only be run for brand new sites.
async fn initialize_local_site_2022_10_10(
  pool: &DbPool,
  settings: &Settings,
) -> Result<(), LemmyError> {
  info!("Running initialize_local_site_2022_10_10");

  // Check to see if local_site exists
  if LocalSite::read(pool).await.is_ok() {
    return Ok(());
  }
  info!("No Local Site found, creating it.");

At this point I gave up because I couldn’t really tell why LocalSite::read(pool).await.is_ok() was, well…not ok.