fix: Support for a custom docker base image in dockerfile/__init__.py.

[mqk]

What does this PR address?

The 1.0.11 release broke the ability to build bentos that are based on a custom docker base image, as described in #3325. In this PR I'm trying to fix that problem. I went down the path of setting docker.distro to a sentinel value ("custom_base_image"). I have no idea if this is a palatable solution, and I won't be offended if you don't think so and prefer something else.

Fixes #3325

Before submitting:

• Does the Pull Request follow Conventional Commits specification naming? Here are GitHub's
  guide on how to create a pull request.
• Does the code follow BentoML's code style, both make format and make lint script have passed (instructions)?
  (make lint failed, but I don't think it has anything to do with these changes.)

[codecov]

Codecov Report

Merging #3328 (0ad4888) into main (7a8ee5f) will decrease coverage by 1.57%.
The diff coverage is 50.00%.

❗ Current head 0ad4888 differs from pull request most recent head 64fc0d9. Consider uploading reports for the commit 64fc0d9 to get more accurate results

@@            Coverage Diff             @@
##             main    #3328      +/-   ##
==========================================
- Coverage   32.91%   31.33%   -1.58%     
==========================================
  Files         109      109              
  Lines        9917     9912       -5     
  Branches     1710     1629      -81     
==========================================
- Hits         3264     3106     -158     
- Misses       6424     6611     +187     
+ Partials      229      195      -34     

Impacted Files	Coverage Δ
...internal/container/frontend/dockerfile/__init__.py	69.76% <25.00%> (-2.74%)	⬇️
src/bentoml/_internal/bento/build_config.py	50.65% <75.00%> (-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 16 more

[mqk]

Closing this PR, since #3329 accomplishes the same thing (but better).