Show Docker icon for files with Dockerfile extension

[TeamTamoad]

There are some situation that developers need to put multiple Dockerfiles in the same directory. This is one of the recommended naming convention.

Before

After

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TODO

• Use cargo fmt
• Add necessary tests
• Add changelog entry
• Update default config/theme in README (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

[codecov-commenter]

Codecov Report

Merging #652 (6551eb6) into master (46f596f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #652   +/-   ##
=======================================
  Coverage   86.56%   86.56%           
=======================================
  Files          40       40           
  Lines        4377     4378    +1     
=======================================
+ Hits         3789     3790    +1     
  Misses        588      588           

Impacted Files	Coverage Δ
src/icon.rs	97.73% <100.00%> (+<0.01%)	⬆️

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[meain]

Hey, thanks for the PR. I just merged in another PR which seems to create a conflict in the CHANGELOG file. Could you push your changes rebased on top of current master.

[TeamTamoad]

Hey, thanks for the PR. I just merged in another PR which seems to create a conflict in the CHANGELOG file. Could you push your changes rebased on top of current master.

Done!