I'm using symfony 4 and this package. I followed install instructions and mimicked your example in the flex tree of the project. Despite having the variable in .env, I get this error:
{"type":"https://tools.ietf.org/html/rfc2616#section-10","title":"An error occurred","detail":"Environment variable not found: \u0022JWT_PRIVATE_KEY_PATH\u0022.","trace":[{"namespace":"","short_class":"","class":"","type":"","function":"","file":"/srv/api/vendor/symfony/dependency-injection/EnvVarProcessor.php","line":76,"args":[]},{"namespace":"Symfony\Component\DependencyInjection","short_class":"EnvVarProcessor","class":"Symfony\Component\DependencyInjection\EnvVarProcessor","type":"-\u003E","function":"getEnv","file":"/srv/api/vendor/symfony/dependency-injection/Container.php","line":394,"args":[["string","string"],["string","JWT_PRIVATE_KEY_PATH"],["object","Closure"]]},{"namespace":"Symfony\Component\DependencyInjection","short_class":"Container","class":"Symfony\Component\DependencyInjection\Container","type":"-\u003E","function":"getEnv","file":"/srv/api/var/cache/dev/Container8nzdbgj/getLexikJwtAuthentication_EncoderService.php","line":17,"args":[["string","string:JWT_PRIVATE_KEY_PATH"]]},{"namespace":"","short_class":"","class":"","type":"","function":"require","file":"/srv/api/var/cache/dev/Container8nzdbgj/srcDevDebugProjectContainer.php","line":411,"args":[["string","/srv/api/var/cache/dev/Container8nzdbgj/getLexikJwtAuthentication_EncoderService.php"]]},{"namespace":"Container8nzdbgj","short_class":"srcDevDebugProjectContainer","class":"Container8nzdbgj\srcDevDebugProjectContainer","type":"-\u003E","function":"load","file":"/srv/api/var/cache/dev/Container8nzdbgj/getLexikJwtAuthentication_JwtManagerService.php","line":12,"args":[["string","getLexikJwtAuthentication_EncoderService.php"]]},{"namespace":"","short_class":"","class":"","type":"","function":"require","file":"/srv/api/var/cache/dev/Container8nzdbgj/srcDevDebugProjectContainer.php","line":411,"args":[["string","/srv/api/var/cache/dev/Container8nzdbgj/getLexikJwtAuthentication_JwtManagerService.php"]]},{"namespace":"Container8nzdbgj","short_class":"srcDevDebugProjectContainer","class":"Container8nzdbgj\srcDevDebugProjectContainer","type":"-\u003E","function":"load","file":"/srv/api/var/cache/dev/Container8nzdbgj/getLexikJwtAuthentication_Security_Guard_JwtTokenAuthenticatorService.php","line":16,"args":[["string","getLexikJwtAuthentication_JwtManagerService.php"]]},{"namespace":"","short_class":"","class":"","type":"","function":"require","file":"/srv/api/var/cache/dev/Container8nzdbgj/srcDevDebugProjectContainer.php","line":411,"args":[["string","/srv/api/var/cache/dev/Container8nzdbgj/getLexikJwtAuthentication_Security_Guard_JwtTokenAuthenticatorService.php"]]},{"namespace":"Container8nzdbgj","short_class":"srcDevDebugProjectContainer","class":"Container8nzdbgj\srcDevDebugProjectContainer","type":"-\u003E","function":"load","file":"/srv/api/var/cache/dev/Container8nzdbgj/getSecurity_Firewall_Map_Context_ApiService.php","line":17,"args":[["string","getLexikJwtAuthentication_Security_Guard_JwtTokenAuthenticatorService.php"]]},{"namespace":"","short_class":"","class":"","type":"","function":"require","file":"/srv/api/var/cache/dev/Container8nzdbgj/srcDevDebugProjectContainer.php","line":411,"args":[["string","/srv/api/var/cache/dev/Container8nzdbgj/getSecurity_Firewall_Map_Context_ApiService.php"]]},{"namespace":"Container8nzdbgj","short_class":"srcDevDebugProjectContainer","class":"Container8nzdbgj\srcDevDebugProjectContainer","type":"-\u003E","function":"load","file":"/srv/api/var/cache/dev/Container8nzdbgj/srcDevDebugProjectContainer.php","line":1065,"args":[["string","getSecurity_Firewall_Map_Context_ApiService.php"]]},{"namespace":"Container8nzdbgj","short_class":"srcDevDebugProjectContainer","class":"Container8nzdbgj\srcDevDebugProjectContainer","type":"-\u003E","function":"Container8nzdbgj\{closure}","file":"/srv/api/vendor/symfony/dependency-injection/ServiceLocator.php","line":64,"args":[]},{"namespace":"Symfony\Component\DependencyInjection","short_class":"ServiceLocator","class":"Symfony\Component\DependencyInjection\ServiceLocator","type":"-\u003E","function":"get","file":"/srv/api/vendor/symfony/security-bundle/Security/FirewallMap.php","line":83,"args":[["string","security.firewall.map.context.api"]]},{"namespace":"Symfony\Bundle\SecurityBundle\Security","short_class":"FirewallMap","class":"Symfony\Bundle\SecurityBundle\Security\FirewallMap","type":"-\u003E","function":"getFirewallContext","file":"/srv/api/vendor/symfony/security-bundle/Security/FirewallMap.php","line":54,"args":[["object","Symfony\Component\HttpFoundation\Request"]]},{"namespace":"Symfony\Bundle\SecurityBundle\Security","short_class":"FirewallMap","class":"Symfony\Bundle\SecurityBundle\Security\FirewallMap","type":"-\u003E","function":"getFirewallConfig","file":"/srv/api/vendor/symfony/security-bundle/EventListener/FirewallListener.php","line":44,"args":[["object","Symfony\Component\HttpFoundation\Request"]]},{"namespace":"Symfony\Bundle\SecurityBundle\EventListener","short_class":"FirewallListener","class":"Symfony\Bundle\SecurityBundle\EventListener\FirewallListener","type":"-\u003E","function":"onKernelRequest","file":null,"line":null,"args":[["object","Symfony\Component\HttpK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