test/remote: Allow override of base image (#9734)

ubuntu:22.04 bumped the ppc64el baseline to POWER9, rendering this
unusable on existing POWER8 systems.  This allows customization similar
to the top-level Dockerfile.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

test/remote/Dockerfile

@@ -1,9 +1,11 @@
+ARG BASE_IMAGE=docker.io/library/ubuntu:22.04
+
 FROM golang:1.18 AS go
 
 RUN go install github.com/mattn/goreman@latest && \
     go install github.com/kisielk/godepgraph@latest
 
-FROM ubuntu:22.04
+FROM $BASE_IMAGE
 
 ENV DEBIAN_FRONTEND=noninteractive
 RUN  apt-get update && apt-get install --no-install-recommends -y \