Your Dockerfile consists of multiple stages, which is also called multi-stage build.

Each FROM statement is a new stage and new image. In your case you have 2 stages:

• builder where you build you app and install curl
• second stage which copies /usr/src/app from builder stage

In this case second FROM node:12-alpine statement will contain only basic alpine packages, node tools and /usr/src/app which you have copied from the first stage.

If you want to have curl in your final image you need to install curl in second stage (after second FROM node:12-alpine):

FROM node:12-alpine AS builder

# Variables from outside
ARG NODE_ENVIRONMENT=development
ENV NODE_ENV=$NODE_ENVIRONMENT

# Create app directory
WORKDIR /usr/src/app

# Do not install 

# Install app dependencies
COPY package*.json ./
RUN npm install

# Bundle app source
COPY . .

# Build Stage 2
# Take the build from the previous stage
FROM node:12-alpine

#Install curl
RUN apk update && apk add curl

WORKDIR /usr/src/app
COPY --from=builder /usr/src/app /usr/src/app

# run the application
EXPOSE 50005 9183

CMD [ "npm", "run", "start:docker" ]

As it was mentioned in comments you can test it by running docker container directly - no need to run pod in k8s cluster:

docker build -t image . && docker run -it image sh -c 'which curl'

It is common to use multi-stage build for applications implemented in compiled programming languages.

1. In the first stage you install all necessary dev tools and compilers and then compile sources into a binary file. Since you don't need and probably don't want sources and developer's tools in a production image you should create a new stage.
2. In the second stage you copy compiled binary file and run it as CMD or ENTRYPOINT. This way your image contains only executable code, which makes them smaller.