Add nonroot user to pipeline's build-base image

Prior to this commit the git-init image only provided a root
user, meaning git clones had to be performed as root.

After this commit the git-init image provides a nonroot user
with UID and GID of 65532 and a home directory of /home/nonroot.
Utilizing this UID results in creds-init credentials being placed
in /home/nonroot/.ssh when disable-home-env-overwrite is "true",
and results in the cloned files being owned by UID 65532.

The HOME environment variable is left untouched because the default
user in the image remains `root`.

images/Dockerfile

@@ -1,5 +1,7 @@
 FROM alpine:3.11
-
+
+RUN addgroup -S -g 65532 nonroot && adduser -S -u 65532 nonroot -G nonroot
+
 RUN apk add --update git git-lfs openssh-client \
     && apk update \
     && apk upgrade