Dockerfile
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RUN executes command(s) in a new layer and creates a new image. E.g., it is often used for installing software packages.
CMD sets default command and/or parameters, which can be overwritten from command line when docker container runs. Therefore CMD is executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored.
ENTRYPOINT configures a container that will run as an executable. It is similar to CMD but command and parameters are not ignored when Docker container runs with command line parameters.
All three instructions (RUN, CMD and ENTRYPOINT) can be specified in shell form or exec form:
in the shell form the instruction is executed calling the shell /bin/sh -c <command>
in the exec form the instruction is executed in an executable directly, and shell processing does not happen.
exec form can be sued to run bash (or any other interpreter but sh)
exec form is preferred for CMD and ENTRYPOINT instructions.
For example RUN has two forms:
RUN <command> (shell form)
RUN ["executable", "param1", "param2"] (exec form)
Use RUN instructions to build your image by adding layers on top of initial image.
Prefer ENTRYPOINT to CMD when building executable Docker image and you need a command always to be executed. Additionally use CMD if you need to provide extra default arguments that could be overwritten from command line when docker container runs.
Choose CMD if you need to provide a default command and/or arguments that can be overwritten from command line when docker container runs.