我的目录仅包含两个文件，Dockerfile并且sayhello.sh：

.
├── Dockerfile
└── sayhello.sh

在Dockerfile读

FROM alpine
COPY sayhello.sh sayhello.sh
CMD ["sayhello.sh"]

并且sayhello.sh仅包含

echo hello

在Dockerfile成功生成：

kurtpeek@Sophiemaries-MacBook-Pro ~/d/s/trybash> docker build --tag trybash .
Sending build context to Docker daemon 3.072 kB
Step 1/3 : FROM alpine
 ---> 665ffb03bfae
Step 2/3 : COPY sayhello.sh sayhello.sh
 ---> Using cache
 ---> fe41f2497715
Step 3/3 : CMD sayhello.sh
 ---> Using cache
 ---> dfcc26c78541
Successfully built dfcc26c78541

但是，如果尝试run这样做，则会executable file not found in $PATH出现错误：

kurtpeek@Sophiemaries-MacBook-Pro ~/d/s/trybash> docker run trybash
container_linux.go:247: starting container process caused "exec: \"sayhello.sh\": executable file not found in $PATH"
docker: Error response from daemon: oci runtime error: container_linux.go:247: starting container process caused "exec: \"sayhello.sh\": executable file not found in $PATH".
ERRO[0001] error getting events from daemon: net/http: request canceled

是什么原因造成的？（我记得debian:jessie以类似的方式在基于图像的脚本中运行脚本，所以也许它是Alpine特有的）？