ci: pass docker push input as a string, not a boolean

Gitea's act_runner mangles boolean workflow_call/dispatch inputs passed
from an expression -- they arrive as false regardless of value. Declare
`push` as a string ("true"/"false") and compare with == 'true' so the
builder image is pushed on non-PR events again.

Dockerfile.builder

@@ -1,4 +1,4 @@
-# KPN++ Builder Image  (CI: pipeline trigger)
+# KPN++ Builder Image  (CI: pipeline trigger v2)
 # Pre-built image with GCC, CMake, Ninja, and Python dev headers for building and testing KPN++
 # Build: docker build -f Dockerfile.builder -t gitea.tourolle.paris/dtourolle/kpnpp-builder:latest .
 # Push:  docker push gitea.tourolle.paris/dtourolle/kpnpp-builder:latest