ci: make container build handle being built as 'root' correctly

ops/devcontainer/Dockerfile

@@ -12,10 +12,14 @@ RUN GOBIN=/usr/local/bin go install git.offline-twitter.com/offline-labs/gocheck
 
 # Create a user in the container with the same UID as on the host machine, to avoid ownership conflicts.
 # The user gets sudo of course.
+#
+# If the host user is `root` (uid = 0), we skip creating a new user, because it will fail otherwise.
 ARG USERNAME
 ARG UID
 ARG GID
-RUN addgroup -g ${GID} ${USERNAME}
-RUN adduser -D -u ${UID} -G ${USERNAME} ${USERNAME}
-RUN echo "${USERNAME} ALL=(ALL) NOPASSWD:ALL" > /etc/sudoers.d/${USERNAME}
+RUN if [ "${UID}" -ne 0 ]; then \
+    addgroup -g "${GID}" "${USERNAME}" && \
+    adduser -D -u "${UID}" -G "${USERNAME}" "${USERNAME}" && \
+    echo "${USERNAME} ALL=(ALL) NOPASSWD:ALL" > /etc/sudoers.d/"${USERNAME}"; \
+    fi
 USER ${USERNAME}