Make var/cache ephemeral

[karniv00l]

Currently whole var/cache directory is mounted and shared between 3 containers:

• app
• consumer
• cron

Only app is clearing and warming up the cache, but the problem is that there is no guarantee that app will actually start and finish clearing cache first before other containers will read it. This may cause issues like: #417

There is not much need to keep caches / sessions between restarts right now.

This change overrides app/var volume with /app/var/cache volume which effectively creates an empty directory in the container on every start. Also I don't see a need for warming up cache for each container.

[codecov]

Codecov Report

Merging #420 (c48c087) into master (12681c9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #420   +/-   ##
=========================================
  Coverage     99.38%   99.38%           
  Complexity     1863     1863           
=========================================
  Files           297      297           
  Lines          5871     5871           
=========================================
  Hits           5835     5835           
  Misses           36       36           

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