fix: promote multi-stage build args to ENV so they propagate across stages

[maxim-voronko]

Build args declared in the wpr_dependencies stage (WPR_FILES_DIR, WPR_APP_INSTALL_DIR, WPR_USER_NAME) did not reach the later wpr_installer and wpr_service stages. An ARG goes out of scope at the end of the stage where it is declared, so in the child stages these variables resolved to empty.

This reproduces on build engines that do not inherit parent-stage ARG defaults (Kaniko, buildah, classic builder)

The fix promotes these three to ENV in the base stage. Unlike ARG, ENV is inherited across FROM, so the values reach all stages on any build engine, without redeclaring each ARG per stage. Applied to both Dockerfile and Dockerfile.redhat. Dockerfile.ubuntu-prebuilt already declares its args per stage and was not affected.