fix: don't overwrite user base image

[aarnphm]

Fixes #3325, a regression introduced by #3164

Signed-off-by: Aaron Pham 29749331+aarnphm@users.noreply.github.com

[codecov]

Codecov Report

Merging #3329 (5642542) into main (970c5b2) will decrease coverage by 1.57%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main    #3329      +/-   ##
==========================================
- Coverage   32.91%   31.33%   -1.58%     
==========================================
  Files         109      109              
  Lines        9917     9908       -9     
  Branches     1710     1628      -82     
==========================================
- Hits         3264     3105     -159     
- Misses       6424     6609     +185     
+ Partials      229      194      -35     

Impacted Files	Coverage Δ
...toml/_internal/server/grpc/servicer/v1/__init__.py	0.00% <ø> (ø)
src/bentoml/_internal/container/generate.py	67.79% <50.00%> (+0.55%)	⬆️
src/bentoml/_internal/bento/build_config.py	50.65% <100.00%> (-4.49%)	⬇️
src/bentoml/_internal/container/__init__.py	44.79% <0.00%> (-32.30%)	⬇️
src/bentoml/_internal/utils/metrics.py	23.33% <0.00%> (-30.00%)	⬇️
src/bentoml/_internal/container/docker.py	34.48% <0.00%> (-27.59%)	⬇️
src/bentoml/_internal/container/base.py	46.72% <0.00%> (-16.83%)	⬇️
src/bentoml/_internal/io_descriptors/pandas.py	33.77% <0.00%> (-9.19%)	⬇️
src/bentoml/_internal/io_descriptors/base.py	79.54% <0.00%> (-9.10%)	⬇️
src/bentoml/_internal/bento/bento.py	73.61% <0.00%> (-6.25%)	⬇️
... and 17 more