Updates for r2 and celery version correction.

[jrespeto]

The docker files are set for python:3.6-slim.
Celery - Python 3.6: Celery 5.1 or earlier.

I tested with python:3.7 and ran into more errors and dep issues.

build with no errors and tested with the test file testbin-puts-mips

> docker-compose exec worker bash
root@6b2c045eb59c:/home/lisa# r2 -V
5.6.4  r2
5.6.4  r_anal
5.6.4  r_lib
5.6.4  r_egg
5.6.4  r_asm
5.6.4  r_bin
5.6.4  r_cons
5.6.4  r_flag
5.6.4  r_core
5.6.4  r_crypto
5.6.4  r_bp
5.6.4  r_debug
5.6.4  r_main
5.6.4  r_hash
5.6.4  r_fs
5.6.4  r_io
5.6.4  r_magic
5.6.4  r_parse
5.6.4  r_reg
5.6.4  r_sign
5.6.4  r_search
5.6.4  r_syscall
5.6.4  r_util

root@6b2c045eb59c:/home/lisa# celery --version
5.1.2 (sun-harmonics)
root@6b2c045eb59c:/home/lisa#

[C0nneR]

testbin-puts-arm failed