Need clarification on command in Dockerfile?

I am trying to run jar file through open source Dkron scheduler. I am need to modify docker file to install java in container. Please clarify me what copy command means here

FROM golang:1.14
LABEL maintainer="Victor Castell <victor@victorcastell.com>"

EXPOSE 8080 8946

RUN mkdir -p /app
WORKDIR /app

ENV GO111MODULE=on
COPY go.mod go.mod
COPY go.sum go.sum            #here
RUN go mod download

COPY . .
RUN go install ./...

CMD ["dkron"]

It also has dockerfile.hub which contains this

FROM alpine
MAINTAINER Victor Castell <victor@victorcastell.com>

ENV DKRON_VERSION 0.11.0

RUN set -x \
	&& buildDeps='bash ca-certificates openssl' \
	&& apk add --update $buildDeps \
	&& rm -rf /var/cache/apk/* \
	&& mkdir -p /opt/local/dkron \
	&& wget -O /opt/local/dkron/dkron.tar.gz https://github.com/victorcoder/dkron/releases/download/v${DKRON_VERSION}/dkron_${DKRON_VERSION}_linux_amd64.tar.gz \
	&& cd /opt/local/dkron \
	&& tar -xzf dkron.tar.gz \
	&& rm /opt/local/dkron/dkron.tar.gz

EXPOSE 8080 8946

ENV SHELL /bin/bash
WORKDIR /opt/local/dkron

ENTRYPOINT ["/opt/local/dkron/dkron"]

CMD ["--help"]

Also what changes need to be done to make java file run in the container? Thank you.

> The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

this is from the Docker documentation: https://docs.docker.com/engine/reference/builder/

> Please clarify me what copy command means here
> ...
>
> COPY . .

The copy step takes files from the build context and copies them into a layer of the resulting image. The build context is passed from the client to the docker engine as the one arg in the build command, typically a . to indicate the current directory is sent as the build context.

You can change the source to be another stage during a multi-stage build, allowing you to transfer build artifacts from a compiler environment to a runtime environment you wish to deploy. And you can change it to pull files from another image. Both of these are done with the --from option to the copy step.

Note that this command only works one way, the destination is always the image you are building, and since the source was historically always the build context, the scope on the source and destination are not included in the syntax. E.g. docker shortened COPY build-context:. container:. to just COPY . ..

Important links in the documention:

• copy step: https://docs.docker.com/engine/reference/builder/#copy
• build context: https://docs.docker.com/engine/reference/builder/#usage