sql server in docker using 15gb of disk space
The database files could be using those 15gb even in an newly created and empty database.

You can check the size of the databases using SSMS:



Or check the size of all databases at once using Alex Aza's code or some other code you like from SQL Server 2008: How to query all databases sizes? which is the source of this sample:

with fs
as
(
    select database_id, type, size * 8.0 / 1024 size
    from sys.master_files
)
select 
    name,
    (select sum(size) from fs where type = 0 and fs.database_id = db.database_id) DataFileSizeMB,
    (select sum(size) from fs where type = 1 and fs.database_id = db.database_id) LogFileSizeMB
from sys.databases db


If you see a database that's bigger than expected, you can use DBCC SHRINKFILE (Transact-SQL) to reduce it. Using this command is ok in your specific situation though and not a recommendation to any situation of a big database file.