Change order of operations in Dockerfile

[bain3]

Hi,
I was tinkering with the code a little and noticed that the Dockerfile could be optimized a little to take advantage of docker's caching.

I've split the run command so all the downloads should only depend on the requirements.txt file and not on the whole codebase.

[codecov-commenter]

Codecov Report

Base: 93.93% // Head: 94.09% // Increases project coverage by +0.16% 🎉

Coverage data is based on head (90b8e2f) compared to base (04bd859).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   93.93%   94.09%   +0.16%     
==========================================
  Files           6        8       +2     
  Lines         742      847     +105     
  Branches      132      154      +22     
==========================================
+ Hits          697      797     +100     
- Misses         31       34       +3     
- Partials       14       16       +2     

Impacted Files	Coverage Δ
webssh/utils.py	97.97% <0.00%> (ø)
webssh/__init__.py	50.00% <0.00%> (ø)

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