Werk #6836: Fix Docker crashes

Component	Checks & agents
Title	Fix Docker crashes
Date	Nov 5, 2018
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0b1 Not yet released Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b1 Not yet released Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b1 Not yet released Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b1 Not yet released Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b1 Not yet released Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.5.0p8 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

Work around missing 'Repository' key if Repository Tags are not defined in image inspect output. Work around missing values if values are zero in 'docker system df'. Do not crash upon empty section in case of missing permissions.

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