write dockerfile for multiple gomod projects

Your Dockerfile is too nested. Since your go build relies on relative paths – paths that are in parent directories – a docker build . will not see any parent-directory files.

Move the Dockerfile to the top e.g.

Dockerile
lib/

and update to build the nested build directory:

FROM golang:1.15

WORKDIR /go/src/app

# just need to copy lib tree
COPY lib .  

# working from here now
WORKDIR /go/src/app/lib/golang/example/golang 

RUN go env -w GO111MODULE=auto    
RUN go env -w GOPROXY=https://goproxy.io,direct

RUN go get -d -v ./...
RUN go install -v ./...

CMD ["app"]