docs: document testcontainers properties file

[mdelapenya]

What does this PR do?

It adds the recently supported .testcontainers.properties file to the Docs.

I basically copied Java's version for this file (https://www.testcontainers.org/features/configuration/), removing anything not related to this project

Why is it important?

Keep docs up-to-date!

How to test this PR?

pip install -r requirements.txt 
mkdocs serve
open http://127.0.0.1:8000/features/custom_configuration/

[codecov[bot]]

Codecov Report

Merging #375 (209f8a9) into master (cad4370) will increase coverage by 2.07%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage   62.09%   64.16%   +2.07%     
==========================================
  Files          15       18       +3     
  Lines        1042     1108      +66     
==========================================
+ Hits          647      711      +64     
  Misses        293      293              
- Partials      102      104       +2     

Impacted Files	Coverage Δ
reaper.go	79.31% <0.00%> (-0.69%)	:arrow_down:
docker.go	64.60% <0.00%> (-0.31%)	:arrow_down:
generic.go	64.70% <0.00%> (ø)
wait/host_port.go	56.14% <0.00%> (ø)
mounts.go	100.00% <0.00%> (ø)
wait/exec.go	92.00% <0.00%> (ø)
docker_mounts.go	92.59% <0.00%> (ø)
compose.go	75.18% <0.00%> (+0.55%)	:arrow_up:
container.go	88.37% <0.00%> (+3.07%)	:arrow_up:

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[mdelapenya]

@bsideup @kiview I'd need your wise eyes in this PR adding docs to the REMOTE_HOST feature 🙏

[mdelapenya]

I'm going to close this issue as we'd like to have more consistent docs for the testcontainers properties file. Will open a new PR with that