In the meantime I googled for utf8mb4_general_ci ghost and found this post. Based on an answer there I ran the following query
MariaDB [(none)]> SELECT DEFAULT_CHARACTER_SET_NAME, DEFAULT_COLLATION_NAME FROM INFORMATION_SCHEMA.SCHEMATA WHERE SCHEMA_NAME='[snipped]';
+----------------------------+------------------------+
| DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME |
+----------------------------+------------------------+
| latin1 | latin1_swedish_ci |
+----------------------------+------------------------+
This looked wrong. So I decided to export my website to JSON, drop all tables in the database and then (as suggested in the aforementioned post) run
ALTER DATABASE `[snipped]` DEFAULT CHARACTER SET utf8mb4 DEFAULT COLLATE utf8mb4_general_ci;
After this I restarted my docker container and the members table now has a status column. So the ALTER DATABASE statement changed something important.
I imported the JSON back into the new website and all seems to work fine. The Add yourself as a member to test button also works.
Thanks Hannah for pointing me in the right direction.