How to fix the error "/bin/sh: 1: source: not found"?

I am installing nodejs inside a docker container and have this line in the Dockerfile:

RUN mkdir -p $NVM_DIR
RUN curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.7/install.sh | bash
RUN source $NVM_DIR/nvm.sh

But when I run the container I get this error:

 => ERROR [node base 11/15] RUN source /usr/local/nvm/nvm.sh                                                                                                                          0.2s

 > [node base 11/15] RUN source /usr/local/nvm/nvm.sh:
#0 0.227 /bin/sh: 1: source: not found             # <<<<<<<<<<<<<<< the error

failed to solve: executor failed running [/bin/sh -c source $NVM_DIR/nvm.sh]: exit code: 127

What causes the error source: not found?

Since bash is installed in the container, I suspected that the /bin/sh prefix means that bash is not used as shell. After a while I found out how to set the shell to bash:

SHELL ["/bin/bash", "-c"]  # <<<<<<<<< add this line
RUN mkdir -p $NVM_DIR
RUN curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.7/install.sh | bash
RUN source $NVM_DIR/nvm.sh

The SHELL ["/bin/bash", "-c"] line tells docker to use bash as shell and the -c tells bash to run the following command. So now the source command is found and the error is fixed.