Add non-root user

Dockerfile

@@ -7,5 +7,12 @@ WORKDIR /usr/src
 # Copy any source file(s) required for the action.
 COPY entrypoint.sh .
 
+# Create a non-root user and switch to it.
+RUN addgroup -S actiongroup && adduser -S actionuser -G actiongroup && \
+    chown -R actionuser:actiongroup /usr/src && \
+    chmod +x /usr/src/entrypoint.sh
+
+USER actionuser
+
 # Configure the container to be run as an executable.
 ENTRYPOINT ["/usr/src/entrypoint.sh"]