- Verify file path and file contents.
- Verify you can do a straight input to stdout or file output with no filters.
What I am reading is you are failing step #2. If that's the case then this is probably a docker question which I can't answer. Maybe something to do with mounting or volumes. But you said you are doing something similar already so I don't think you would miss that part.
Below is what I would use to test.
input {
file {
path => "/path/*.json"
start_position => "beginning"
sincedb_path => "/dev/null"
}
}
output {
stdout {}
}