Use prebuilt Xpra base image in Dockerfile

[jacob-curley-fnal]

Mitigates errors with new versions of Xpra being loaded. File transfers that work on v6.4.4 are broken on v6.5.x

[github-actions]

Code Coverage Report - 46 of 117 lines covered ( ⛔ 39.32%)

src - 46 of 117 lines covered ( ⛔ 39.32%)

src/ap_python_starter_kit - 46 of 117 lines covered ( ⛔ 39.32%)

src/ap_python_starter_kit/init.py - 0 of 2 lines covered ( ⛔ 0.00%)

⛔ This file is missing coverage.

src/ap_python_starter_kit/acsys_client.py - 9 of 28 lines covered ( ⛔ 32.14%)

Uncovered lines: ⚠️ 18-19, 21-24, 26-28, 30, 34, 36-38, 40-41, 45-46, 48

src/ap_python_starter_kit/gui.py - 16 of 65 lines covered ( ⛔ 24.62%)

Uncovered lines: ⚠️ 12-13, 20-23, 25-26, 28-31, 33-34, 36, 39, 42, 45, 51, 58-59, 61-65, 67-68, 70-73, 75, 77-82, 84-85, 87-88, 90-91, 95-98

src/ap_python_starter_kit/main.py - 21 of 22 lines covered ( ✅ 95.45%)

Uncovered lines: ⚠️ 47