pg_restore: error: could not read from input file: end of file
You really have two questions here.  First you have a corrupted tar file.  I'd blame that on your usage of docker.  If you run tar -tf ... on the docker side before moving the file, was it corrupted there?

Second, you don't want to restore data, you want to merge it.  pg_dump/pg_restore doesn't do that.  You will have to figure out what you want to do (overwrite, aggregate, assign new PKs and just insert them as new rows) and then implement that. You likely want to use COPY or \copy to get the data out of the first system so the second one can see it.  After that, it is really up to you what you do with it.  You can implement the merging in python or Perl or SQL (after \copy into staging tables) or whatever your favorite tool is.