Add /start shim for containers using old entrypoint (#3663)

Author: itzg

Dockerfile

@@ -66,6 +66,10 @@ STOPSIGNAL SIGTERM
 ENV TYPE=VANILLA VERSION=LATEST EULA="" UID=1000 GID=1000 LC_ALL=en_US.UTF-8
 
 COPY --chmod=755 scripts/start* /image/scripts/
+
+# Backward compatible shim for those with legacy entrypoint
+RUN echo "#!/bin/sh\nexec /image/scripts/start\n" > /start && chmod +x /start
+
 COPY --chmod=755 scripts/auto/* /image/scripts/auto/
 COPY --chmod=755 files/shims/ /usr/local/bin/
 COPY --chmod=755 files/* /image/