Support docker build comparable --ssh switch

[jimsnab]

Actual behavior I've not found a way to securely use the host's ssh keys from within a Dockerfile using kaniko.

Expected behavior Provide compatibility with docker build --ssh, which enables the docker file to run commands that use ssh keys.

To Reproduce Steps to reproduce the behavior:

1. Run kaniko and specify --ssh. It is not recognized as a switch (expected; it is not documented to be supported).

Additional Information

• Dockerfile Please provide either the Dockerfile you're trying to build or one that can reproduce this error.

Ordinary Dockerfile fragment that builds a go container and uses a private scm

COPY . .
RUN --mount=type=ssh go mod download
RUN --mount=type=ssh go build -o /opt/apiserver/apiserver -buildvcs=false

• Build Context Please provide or clearly describe any files needed to build the Dockerfile (ADD/COPY commands)

• Kaniko Image (fully qualified with digest)

  Triage Notes for the Maintainers

  Description	Yes/No
  Please check if this a new feature you are proposing	- [ Yes ]
  Please check if the build works in docker but not in kaniko	- [ Yes ]
  Please check if this error is seen when you use --cache flag	- [ Yes ]
  Please check if your dockerfile is a multistage dockerfile	- [ Yes ]

[jimsnab]

One way you can repro this is to use a private github.com/username repo in a hello world go project.

Then try building with this... replace the ARGs according to your private ssh url for the repo. Also the executable name below is apiserver.

FROM golang:alpine
RUN apk update && apk add --no-cache git && apk add openssh

# Private repo
ARG PRIVATE_SCM_HOST
ARG PRIVATE_SCM_SSH_PORT

ENV GOPRIVATE=$PRIVATE_SCM_HOST

# SSH Key setup
RUN mkdir -p /root/.ssh && \
    chmod 0700 /root/.ssh && \
    ssh-keyscan -p $PRIVATE_SCM_SSH_PORT -H $PRIVATE_SCM_HOST > /root/.ssh/known_hosts

# Configure https to ssh mapping
RUN git config --global url."ssh://git@$PRIVATE_SCM_HOST:$PRIVATE_SCM_SSH_PORT".insteadOf "https://$PRIVATE_SCM_HOST/scm"

# Create app directory
WORKDIR /opt/apiserver

# Bring source into the container and build
COPY . .
RUN --mount=type=ssh go mod download
RUN --mount=type=ssh go build -o /opt/apiserver/apiserver -buildvcs=false

CMD [ "/opt/apiserver/apiserver" ]

docker build --build-arg PRIVATE_SCM_HOST=github.com/username/repo --build-arg PRIVATE_SCM_SSH_PORT=22 --build-arg --ssh default .