pin Ubuntu version in distroless docker file (#3449)

* Update dockerfile-cli-only-distroless.gpu

* Update README.md

* Update README.md

docker/gpu/README.md

 # Tiny Distroless Dockerfile for LightGBM GPU CLI-only Version
 
-`dockerfile-cli-only-distroless.gpu` - A multi-stage build based on the `nvidia/opencl:devel` (build) and `distroless/cc-debian10` (production) images. LightGBM (CLI-only) can be utilized in GPU and CPU modes. The resulting image size is around 15 MB.
+`dockerfile-cli-only-distroless.gpu` - A multi-stage build based on the `nvidia/opencl:devel-ubuntu18.04` (build) and `distroless/cc-debian10` (production) images. LightGBM (CLI-only) can be utilized in GPU and CPU modes. The resulting image size is around 15 MB.
 
 ---
 

docker/gpu/dockerfile-cli-only-distroless.gpu

@@ -15,7 +15,7 @@
 # CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 # DEALINGS IN THE SOFTWARE.
 
-FROM nvidia/opencl:devel AS build
+FROM nvidia/opencl:devel-ubuntu18.04 AS build
 ARG DEBIAN_FRONTEND=noninteractive
 ARG OPENCL_LIBRARIES=/usr/lib/x86_64-linux-gnu
 ARG OPENCL_INCLUDE_DIR=/usr/include/CL