Modified the Dockerfile to make it follow FHS conventions and fixing #60 bug

[ShibaOnTheMoon]

Copying a binary file inside the /etc folder isn't a good idea. Furthermore, it prevents the use of a volume directly on the folder /etc/dockerbeat.

Only minor changes of the Dockerfile are necessary.

You can find more about the FHS specs here : http://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html

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I've also fixed the the #60 issue (I wanted to create 2 distincts pull requests, but I've made a nistake while pushing my local work... sorry !)

[codecov-io]

Current coverage is 70.57%

Merging #77 into develop will increase coverage by +0.81% as of 41a1ff0

@@            develop     #77   diff @@
=======================================
  Files             7       7       
  Stmts           420     384    -36
  Branches         25      25       
  Methods           0       0       
=======================================
- Hit             293     271    -22
  Partial           4       4       
+ Missed          123     109    -14

Review entire Coverage Diff as of 41a1ff0

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[erwanncloarec]

Thanks, it LGTM.