How to process json logs from docker container

[paulov (Paul Oude Vrielink)]

Hi,

I receive logs from docker containers, these have a field 'log' that is in itself a json message and that contains a subfield 'message' which I'm interested in.

Example:
"log"=>"{"fields":{},"level":"info","@timestamp":1647597990820,"message":"{\"id\":\"c4590000-fdc0-da0b-2c67-08da08c6f119\",\"created_at\":\"2022-03-18T10:06:18.798Z\",\"error\":{\"error\":\"\",\"error_description\":\"\"},\"scanner_information\":{\"hardware_id\":\"PRMC3N-OEM-03-203048\",\"certificate_serial_number\":\"\"}],

I want to filter on the message field with the following filter:

json {
  source => "message"
}

However I get:

:response=>{"index"=>{"_index"=>"document-verification-000001", "_type"=>"_doc", "_id"=>"likYrH8BR6mWLvAxG3SC", "status"=>400, "error"=>{"type"=>"mapper_parsing_exception", "reason"=>"object mapping for [log] tried to parse field [log] as object, but found a concrete value"}}}}

I don't know how to build the filter so I can use the inner 'message' json. I also tried double filters like:

json {
  source => "log"
}

I then don't get the error but also don't get data.
Any help?

[paulov (Paul Oude Vrielink)]

It appears that if I specify in the filter:
json {
source => "message"
}
json {
source => "log"
}

Then it works. The 'log' element is getting json-parsed. However I then run into another problem: the size of the json. Although the 'log' element is about 20k, I get an error indicating a max size of 32769 (2^15):
:exception=>#<LogStash::Json::ParserError: Unexpected end-of-input in VALUE_STRING
at [Source: (byte)"{"fields":{},"level":"info","@timestamp":1647597990820,"message":"{"id":"c4590000-fdc0-da0b-2c67-08da08c6f119","created_at":"2022-03-18T10:06:18.798Z","error":{"error":"","error_description":""},"scanner_information":{"hardware_id":"PRMC3N-OEM-03-203048","certificate_serial_number":""},"document_verification":{"overall_status":"not_passed","auto_checks":{"error":{"error":"","error_description":""},"calculated_risk_value":90,"document_details":{""[truncated 15884 bytes]; line: 1, column: 32769]>}

Anyone know if I can increase this max size? (I can't find anything on this).