Add PHP linter for 7.3

php-lint/7.3/Dockerfile

+FROM php:7.3-alpine
+
+ARG BUILD_DATE
+ARG VCS_REF
+ARG VERSION
+LABEL org.label-schema.build-date=$BUILD_DATE \
+      org.label-schema.name="PHP 7.3 linter" \
+      org.label-schema.description="" \
+      org.label-schema.vcs-ref=$VCS_REF \
+      org.label-schema.vendor="University of Nebraska - Lincoln" \
+      org.label-schema.version=$VERSION \
+      org.label-schema.schema-version="1.0" \
+      maintainer="Alan Nelson <alan.nelson@nebraska.edu>"
+
+RUN apk add --no-cache bash
+
+COPY php-lint docker-entrypoint /usr/local/bin/
+RUN chmod 755 /usr/local/bin/docker-entrypoint /usr/local/bin/php-lint
+
+ENTRYPOINT ["docker-entrypoint"]
+CMD ["--help"]