Building a minimal container for a Go program - go

Question

I want to build a tiny container image from scratch using Buildah to run a Go app.
Apart from the app itself, what other libraries etc need to be included. I am thinking that glibc is needed - is there anything else?
So in summary, I think I am asking "what are all the external dependencies that a compiled Go app needs on Linux?"

Answer 1

#Dave C gave the information to correctly answer this. Using ldd with the test app returned:
[bryon#localhost resttest]$ ldd restest
linux-vdso.so.1 (0x00007fff139fe000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fbad6ce2000)
libc.so.6 => /lib64/libc.so.6 (0x00007fbad691f000)
/lib64/ld-linux-x86-64.so.2 (0x00007fbad6f02000)
[bryon#localhost resttest]$
So for those looking to build a minimal container with Buildah, the BASH script to generate it would look like this:
#!/bin/bash
#
# Run this shell script after you have run the command: "buildah unshare"
#
git clone https://github.com/bryonbaker/resttest.git
cd resttest
go build restest.go
container=$(buildah from scratch)
mnt=$(buildah mount $container)
mkdir $mnt/bin
mkdir $mnt/lib64
buildah config --workingdir /bin $container
buildah copy $container restest /bin/restest
buildah copy $container /lib64/libpthread.so.0 /lib64
buildah copy $container /lib64/libc.so.6 /lib64
buildah copy $container /lib64/ld-linux-x86-64.so.2 /lib64
buildah config --port 8000 $container
#
# This step is not working properly.
# Need to run with podman -p 8000:8000 --entrypoint /bin/restest restest:latest
buildah config --entrypoint /bin/restest $container
buildah commit --format docker $container restest:latest
This generates a 14MB container for a simple microservice! There are no additional files to be worrying about for vulnerabilities etc.
I have a small defect I can't work out on entrypoints so I am overriding the entrypoint on start, but to test it run:
podman -p8000:8000 --entrypoint /bin/restest restest:latest
Then just type the following in a Terminal session:
curl http://localhost:8000
So thanks Dave C!

Answer 2

I know this is quite late answer, but it does tell how to build the slimmiest image for Golang programs. It is based on the question Deployment using image from scratch fails to start
The trick is to build statically linked executable and place it into the empty image called scratch. The image contains just a single file, that exact executable. It is the smallest image possible.
Docker file:
FROM golang:latest as builder
# The Dockerfile expects the source code of the application
# to reside in ./src/ directory
COPY src /src
WORKDIR /src
# Build statically linked file and strip debug information
# The Dockerfile expects the `main` package to be at the root of the module
RUN CGO_ENABLED=0 go build -ldflags="-extldflags=-static -s -w" -o executable
# scratch is an empty image
FROM scratch
# If you need /bin/sh and a few utilities, uncomment
# the following line. It increases the image by 5.5 MB
# FROM alpine:latest
COPY --from=builder /src/executable /executable
# copy other files if needed
ENTRYPOINT ["/executable"]
The Dockerfile expects the source code to be in src directory
<project_root>
|_ Dockerfile
|_ src/
|_ go.mod
|_ package_main.go # file with `package main` and `func main()`
|_ other source files
The command docker build ./ -t my-minimal-go produces the image named my-minimal-go:latest
To prove that it is the minimal image, save it to TAR and study the contents:
docker image save my-minimal-go:latest > my-minimal-go.tar
tar tf my-minimal-go.tar
The contents is something like
84ebda22f9b32043fdcb7bb70c559f0ee91cac60b4b92f1ce424662afec6d4b9.json
e622775ad65d50bc0b9f30e6ce58ee7670f752c63c3ca70caba4f9165efdca80/
e622775ad65d50bc0b9f30e6ce58ee7670f752c63c3ca70caba4f9165efdca80/VERSION
e622775ad65d50bc0b9f30e6ce58ee7670f752c63c3ca70caba4f9165efdca80/json
e622775ad65d50bc0b9f30e6ce58ee7670f752c63c3ca70caba4f9165efdca80/layer.tar
manifest.json
repositories
And to see the list of files in the image:
docker image save my-minimal-go:latest | tar x --wildcards '*layer.tar' -O | tar t
Output:
executable
Just a single file, the minimal image.

Answer 3

I am assuming you have included the app dependencies in your docker image.
You won't require any external dependency to build a docker image. Just base image from Go is sufficient to build and run on Linux machines.
# Start from the latest Go base image
FROM golang:latest
# Set the Current Working Directory inside the container
WORKDIR /app
# Copy go mod and sum files
COPY go.mod go.sum ./
# Download all dependencies. Dependencies will be cached if the go.mod and go.sum files are not changed
RUN go mod download