Add CreatedAtTimestamp and fix CreatedAt docs

[simonha9]

Fixes #2413

- What I did Added CreatedAtTimestamp and fix documentation for CreatedAt.

- How I did it As above.

- How to verify it TestContainerContextWriteJSON in container_test.go

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #4765 (bd36415) into master (cfe18f5) will decrease coverage by 0.01%. Report is 41 commits behind head on master. The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4765      +/-   ##
==========================================
- Coverage   59.62%   59.62%   -0.01%     
==========================================
  Files         287      285       -2     
  Lines       24777    24776       -1     
==========================================
- Hits        14774    14772       -2     
  Misses       9117     9117              
- Partials      886      887       +1