UndefinedVar: Usage of undefined variable '$CSP_STYLE_SRC'

[jaller94]

Variables should be defined before their use

UndefinedVar: Usage of undefined variable '$CSP_STYLE_SRC'
More info: https://docs.docker.com/go/dockerfile/rule/undefined-var/

Reported by GitHub: https://github.com/nordeck/matrix-poll/pull/573/files