build: add Docker entrypoint script

Dockerfile.alpine

@@ -22,10 +22,13 @@ ENV DOCKER_GEN_VERSION=${DOCKER_GEN_VERSION} \
 # Install packages required by the image
 RUN apk add --no-cache --virtual .bin-deps openssl
 
+# Copy the entrypoint script
+COPY /app/docker-entrypoint.sh /app/docker-entrypoint.sh
+
 # Install docker-gen from build stage
 COPY --from=go-builder /build/docker-gen /usr/local/bin/docker-gen
 
 # Copy the license
 COPY LICENSE /usr/local/share/doc/docker-gen/
 
-ENTRYPOINT ["/usr/local/bin/docker-gen"]
+ENTRYPOINT ["/app/docker-entrypoint.sh"]

Dockerfile.debian

@@ -26,10 +26,13 @@ RUN apt-get update \
     && apt-get clean \
     && rm -r /var/lib/apt/lists/*
 
+# Copy the entrypoint script
+COPY /app/docker-entrypoint.sh /app/docker-entrypoint.sh
+
 # Install docker-gen from build stage
 COPY --from=go-builder /build/docker-gen /usr/local/bin/docker-gen
 
 # Copy the license
 COPY LICENSE /usr/local/share/doc/docker-gen/
 
-ENTRYPOINT ["/usr/local/bin/docker-gen"]
+ENTRYPOINT ["/app/docker-entrypoint.sh"]

app/docker-entrypoint.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+set -eu
+
+# run container's CMD if it is an executable in PATH
+command -v -- "$1" >/dev/null 2>&1 || set -- docker-gen "$@"
+
+exec "$@"