Extract 'REMOTE_EXIT_CODE=' from stdin and pass the rest through

filter-stdin.py

#!/usr/bin/env python3
import fileinput, sys

exitCode = 0

for line in fileinput.input():
    if line.startswith('REMOTE_EXIT_CODE='):
        exitCode = int(line.split(sep='=', maxsplit=1)[1])
    else:
        print(line, end='')

sys.exit(exitCode)

Usage and context provided in this comment: balena-io/balena-cli#1535 (comment)