Fail if environment variable doesn't exist
When you use an enviroment variable & forget to set this (e.g. in a context), shell's default is to silently use an empty value instead of returning with an error.
The option -o nounset would always throw a helpful error message when the environment variable doesn't exist/is unset. That option also works for in the shell used in Alpine Linux (which is used for many official Docker images):
$ docker run alpine sh -c 'set -o nounset; echo $TEST_ENV'
sh: TEST_ENV: parameter not set
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