fix(core): improper reading of .testcontainers.properties (#863)

fix #864 

The environment variables were not overridden from the
.testcontainers.properties file for ryuk variables. This causes the
properties file to never actually be used. This commit detects the
environment variable, and if unspecified falls back to the properties
file, and if not specified, defaults to false

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Co-authored-by: David Ankin <daveankin@gmail.com>