I have a simple docker file of

FROM ubuntu:18.04
COPY . /usr/src/app/
COPY docker_files/.bash_aliases /root/
$ docker build -t dock .
Sending build context to Docker daemon  146.9kB
Step 1/3 : FROM ubuntu:18.04
—> 94e814e2efa8
Step 2/3 : COPY . /usr/src/app/
—> bf79eb6c42c1
Step 3/3 : COPY docker_files/.bash_aliases /root/
—> aedc97d5ee8b
Successfully built aedc97d5ee8b
Successfully tagged dock:latest

I can use it:

$ docker run -it dock
bash: git: command not found
root@6a6bec871690:/# ls usr/src/app/
Dockerfile  Gemfile  Gemfile.lock  README.md  docker_files  go  ...
root@6a6bec871690:/# 

and as you see my files were copied and aliases created for root. However git was not found (hence the err msg) and not installed.
How do I install it given my attempt below failed?

root@753b573271d5:/# git
bash: git: command not found
root@753b573271d5:/# apt-get install git
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package git
root@753b573271d5:/# sudo apt-get install git 
bash: sudo: command not found

This Dockerfile works for me

FROM ubuntu:18.04
RUN apt update
RUN apt install -y git

Inside the container

$ which git
/usr/bin/git