warnings: UndefinedVar and LegacyKeyValueFormat in Dockerfile

[rngadam]

v1.8.0-10-g58dd883

 3 warnings found (use docker --debug to expand):
 - UndefinedVar: Usage of undefined variable '$LOG_LEVEL' (line 3)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 3)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 4)