fix: remove circular PYTHONPATH reference in Dockerfile

[dvorak0]

No description provided.

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.13 s (285.5 files/s, 43368.9 lines/s)

Language	files	blank	comment	code
Python	12	542	244	3004
C++	4	60	38	409
Markdown	4	143	6	405
Bourne Shell	10	17	7	163
Dockerfile	1	27	23	159
JSON	2	8	0	57
TOML	1	5	0	52
make	1	18	12	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	36	824	330	4315

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.14 s (251.1 files/s, 38146.1 lines/s)

Language	files	blank	comment	code
Python	12	542	244	3004
C++	4	60	38	409
Markdown	4	143	6	405
Bourne Shell	10	17	7	163
Dockerfile	1	27	23	160
JSON	2	8	0	57
TOML	1	5	0	52
make	1	18	12	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	36	824	330	4316