Dockerfile: add the ARG defaulting within the Dockerfile

Add the default value for `GO_VERSION` within the Dockerfile to
fix build checks warnings that come from an empty variable. This
doesn’t break existing behavior; version control is still managed
by the Makefile
```
 - InvalidDefaultArgInFrom: Default value for ARG \
golang:${GO_VERSION} results in empty or invalid base image name (line 17)
```

Signed-off-by: Feruzjon Muyassarov <[email protected]>

Author: fmuyassarov

Dockerfile

@@ -14,7 +14,7 @@
 
 # Build the manager binary
 ARG GO_VERSION
-FROM golang:${GO_VERSION} AS builder
+FROM golang:${GO_VERSION:-1.24.7} AS builder
 WORKDIR /workspace
 
 # Run this with docker build --build_arg goproxy=$(go env GOPROXY) to override the goproxy