Handle the 'network_mode' key when merging multiple compose files.
Handle the 'network_mode' key when merging multiple compose files.

Fixes docker/compose#2840.

Signed-off-by: Lukas Waslowski <027bf5a5b5299782a54f4c941e5907055cba019d@gmail.com>

alexisbellido/docker.github.io,sanscontext/docker.github.io,shubheksha/docker.github.io,johnstep/docker.github.io,funkyfuture/docker-compose,londoncalling/docker.github.io,anweiss/docker.github.io,londoncalling/docker.github.io,JimGalasyn/docker.github.io,johnstep/docker.github.io,rillig/docker.github.io,jzwlqx/denverdino.github.io,jzwlqx/denverdino.github.io,JimGalasyn/docker.github.io,docker/docker.github.io,joeuo/docker.github.io,hoogenm/compose,joeuo/docker.github.io,danix800/docker.github.io,sanscontext/docker.github.io,sdurrheimer/compose,BSWANG/denverdino.github.io,sanscontext/docker.github.io,troy0820/docker.github.io,denverdino/denverdino.github.io,LuisBosquez/docker.github.io,shubheksha/docker.github.io,troy0820/docker.github.io,bdwill/docker.github.io,menglingwei/denverdino.github.io,danix800/docker.github.io,denverdino/denverdino.github.io,troy0820/docker.github.io,jzwlqx/denverdino.github.io,shubheksha/docker.github.io,thaJeztah/docker.github.io,joaofnfernandes/docker.github.io,joaofnfernandes/docker.github.io,denverdino/docker.github.io,gdevillele/docker.github.io,denverdino/docker.github.io,gdevillele/docker.github.io,joaofnfernandes/docker.github.io,shin-/compose,docker/docker.github.io,JimGalasyn/docker.github.io,thaJeztah/docker.github.io,vdemeester/compose,jzwlqx/denverdino.github.io,thaJeztah/docker.github.io,docker-zh/docker.github.io,londoncalling/docker.github.io,aduermael/docker.github.io,gdevillele/docker.github.io,londoncalling/docker.github.io,rillig/docker.github.io,BSWANG/denverdino.github.io,shin-/docker.github.io,menglingwei/denverdino.github.io,alexisbellido/docker.github.io,denverdino/docker.github.io,phiroict/docker,menglingwei/denverdino.github.io,alexisbellido/docker.github.io,docker/docker.github.io,rillig/docker.github.io,dnephin/compose,dnephin/compose,anweiss/docker.github.io,vdemeester/compose,thaJeztah/compose,gdevillele/docker.github.io,johnstep/docker.github.io,aduermael/docker.github.io,johnstep/docker.github.io,rillig/docker.github.io,phiroict/docker,thaJeztah/compose,BSWANG/denverdino.github.io,shin-/compose,troy0820/docker.github.io,danix800/docker.github.io,JimGalasyn/docker.github.io,denverdino/denverdino.github.io,bdwill/docker.github.io,docker-zh/docker.github.io,schmunk42/compose,schmunk42/compose,jzwlqx/denverdino.github.io,jrabbit/compose,phiroict/docker,joaofnfernandes/docker.github.io,funkyfuture/docker-compose,LuisBosquez/docker.github.io,johnstep/docker.github.io,bdwill/docker.github.io,shin-/docker.github.io,denverdino/denverdino.github.io,phiroict/docker,denverdino/denverdino.github.io,swoopla/compose,joeuo/docker.github.io,shin-/docker.github.io,sdurrheimer/compose,joeuo/docker.github.io,sanscontext/docker.github.io,shubheksha/docker.github.io,joeuo/docker.github.io,docker-zh/docker.github.io,bdwill/docker.github.io,joaofnfernandes/docker.github.io,docker-zh/docker.github.io,thaJeztah/docker.github.io,LuisBosquez/docker.github.io,swoopla/compose,jrabbit/compose,denverdino/docker.github.io,docker/docker.github.io,alexisbellido/docker.github.io,anweiss/docker.github.io,menglingwei/denverdino.github.io,menglingwei/denverdino.github.io,danix800/docker.github.io,docker-zh/docker.github.io,denverdino/docker.github.io,bdwill/docker.github.io,gdevillele/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,shin-/docker.github.io,anweiss/docker.github.io,anweiss/docker.github.io,aduermael/docker.github.io,alexisbellido/docker.github.io,shin-/docker.github.io,LuisBosquez/docker.github.io,hoogenm/compose,LuisBosquez/docker.github.io,shubheksha/docker.github.io,aduermael/docker.github.io,phiroict/docker,BSWANG/denverdino.github.io,BSWANG/denverdino.github.io,docker/docker.github.io,sanscontext/docker.github.io,JimGalasyn/docker.github.io