[Transcoder] restrict ffmpeg process

[TroyKomodo]

Proposed changes

Remove env variables and change the UID and GID of the ffmpeg process.

completes #86 part 3

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] Lint and unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)
• [x] Any dependent changes have been merged and published in downstream modules

[codecov[bot]]

Codecov Report

Merging #94 (192e363) into main (f5c2914) will increase coverage by 0.00%. The diff coverage is 69.56%.

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   73.88%   73.88%           
=======================================
  Files         209      209           
  Lines       15180    15196   +16     
=======================================
+ Hits        11215    11228   +13     
- Misses       3965     3968    +3     

Impacted Files	Coverage Δ
video/transcoder/src/transcoder/job/mod.rs	75.52% <66.66%> (-0.11%)	:arrow_down:
video/transcoder/src/config.rs	89.04% <100.00%> (+0.30%)	:arrow_up:

... and 1 file with indirect coverage changes