Docker build for Go project with GitLab private repositories

I am having some issues getting my build to work with dependencies in private GitLab repositories. All-in-all it is a multistage build, but the stage where I am attempting to build my Go project is listed below. This works locally for me, so there is an issue somewhere getting this working in Docker:

FROM golang:1.16.8-alpine3.14 as BuildStage

RUN apk update && apk add --no-cache git ca-certificates tzdata gcc libc-dev openssh-client bash

RUN mkdir /root/.ssh
RUN ssh-keyscan -H gitlab.com >> ~/.ssh/known_hosts

COPY localRsa /root/.ssh/id_rsa
RUN chmod 0400 /root/.ssh/id_rsa

RUN eval $(ssh-agent -s) && ssh-add /root/.ssh/id_rsa

WORKDIR $GOPATH/src/myproject
COPY . .

ENV GOPRIVATE="gitlab.com/MyGitLabUser"
RUN git config --global url."git@gitlab.com".insteadOf "https://gitlab.com"

RUN go mod download
RUN go mod verify

RUN GOOS=linux GOARCH=amd64 \
    go build -ldflags='-w -s -extldflags "-static"' -tags musl -a -o /go/bin/mybinary

The error message I get:
> go: gitlab.com/MyProject/Sub1/Sub2/some-library@v0.0.6: reading gitlab.com/MyProject/Sub1/Sub2/some-library.git/go.mod at revision v0.0.6: unknown revision v0.0.6

The release definitely exists and is working locally. I am missing a step somewhere.

Update:

Cloning the project works from that stage if I add:

git clone git@gitlab.com:MyProject/Sub1/Sub2/some-library.git

Makes me think I'm missing something in my Go configuration or linking Go with Git.

Issues

After putting together everything that everyone provided here (thanks, by the way). I was able to rework my Dockerfile to do exactly what I needed and get things to work! So first, let me go through and enumerate all of the problems that were in my original submission:

1. Passing in the SSH key is completely unnecessary (see here - hat tip @RakeshGupta)
2. Using go mod download -x helped a lot to search out there for more specific information (hat tip @mh-cbon)
3. I was able to simplify more (see here - hat tip @sytech)
4. One of the big things was a typo that I fixed, so now it's: git config --global url."git@gitlab.com:".insteadOf "https://gitlab.com/"

Updated Dockerfile

FROM golang:1.16.8-alpine3.14 as BuildStage

# setup Git & SSL (for getting dependencies)
RUN apk update && \
    apk add --no-cache git ca-certificates tzdata gcc libc-dev openssh-client && \
    update-ca-certificates

ENV GOPRIVATE="gitlab.com/MyProject"

RUN mkdir -p -m 0600 ~/.ssh && ssh-keyscan gitlab.com >> ~/.ssh/known_hosts
RUN git config --global url."git@gitlab.com:".insteadOf "https://gitlab.com/"

# setup an application user
ENV USER=appuser
ENV UID=10001

RUN adduser --disabled-password \
            --gecos "" \
            --home "/nonexistent" \
            --shell "/sbin/nologin" \
            --no-create-home \
            --uid "${UID}" "${USER}"

# build the project
WORKDIR $GOPATH/src/myproject
COPY . .

# make sure Go knows the packages are private
RUN go env -w GOPRIVATE="gitlab.com/MyProject/*"

# build the binary
RUN --mount=type=ssh go mod download -x && go mod verify
RUN --mount=type=ssh GOOS=linux GOARCH=amd64 go build -ldflags='-w -s -extldflags "-static"' -tags musl -a -o /go/bin/mybinary

Updated Build Command

So that's cute and all, but I need BuildKit with SSH:

eval "$(minikube docker-env)"
DOCKER_BUILDKIT=1 docker build --ssh default -t myservice:latest .