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I am new to Docker. I found that we can set environment variables using the ENV instruction in the Dockerfile. But how does one set Bash aliases for long commands in Dockerfile?

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Basically like you always do, by adding it to the user's .bashrc file:

FROM foo
RUN echo 'alias hi="echo hello"' >> ~/.bashrc

As usual this will only work for interactive shells:

docker build -t test .
docker run -it --rm --entrypoint /bin/bash test hi
/bin/bash: hi: No such file or directory
docker run -it --rm test bash
$ hi
hello

For non-interactive shells you should create a small script and put it in your path, i.e.:

RUN echo -e '#!/bin/bash\necho hello' > /usr/bin/hi && \
    chmod +x /usr/bin/hi

If your alias uses parameters (ie. hi Jim -> hello Jim), just add "$@":

RUN echo -e '#!/bin/bash\necho hello "$@"' > /usr/bin/hi && \
    chmod +x /usr/bin/hi