Feature/build profile

[sbannier]

Changelog: (Fix): When using Docker: only pass a build profile if it has been explicitly specified

By default the ´´´default´´´ profile has been used if no build profile was given while creating the ´´´ConanMultiPackager´´´ for Docker. This causes Conan to always be run with a build profile, which is different to how it is run when not using Docker. Conan also behaves differently for some use cases when a build profile is given.

This pull request also strips down the build profile text passed to the Docker container via environment variables to a bare minimum. The reason for this is explained in the commit message (this change can also be reverted if I missed something)

fixes #595

• [x ] Refer to the issue that supports this Pull Request.
• [x ] If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• [x ] I've read the Contributing guide.
• [ x] I've followed the PEP8 style guides for Python code.

[codecov-commenter]

Codecov Report

Merging #604 (34c149c) into develop (28ae68d) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #604   +/-   ##
========================================
  Coverage    78.60%   78.60%           
========================================
  Files           43       43           
  Lines         5290     5291    +1     
========================================
+ Hits          4158     4159    +1     
  Misses        1132     1132           

Impacted Files	Coverage Δ
cpt/packager.py	87.16% <100.00%> (+0.05%)	:arrow_up:
cpt/profiles.py	91.37% <100.00%> (-0.15%)	:arrow_down:
cpt/ci_manager.py	92.46% <0.00%> (ø)

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