Alternative to using --squash when building docker images on Windows using local files - docker

Question

We have some local installers and zip files that we use to build our docker images. It is easy to get this to work in a Dockerfile:
FROM mcr.microsoft.com/windows/nanoserver
COPY myinstaller.exe .
RUN myinstaller.exe; \
del myinstaller.exe
The problem here is that it produces a layer for the COPY line, which increases the size of the image. A common work-around for this is to have one RUN line, that downloads the file from the Internet, runs commands, and then deletes the installation file. The problem, as written above, is that the installers are on the local filesystem.
I found that there is a --squash command for docker:
docker build --squash -t mytestimage .
This does exactly what I want: It gives me an image without this extra installer file that is not necessary. To run this command, you need to enable experimental features though. There is also an open issue to simply remove this feature:
https://github.com/moby/moby/issues/34565
Is there some alternative way of using local installers in a Dockerfile when running on Windows, that doesn't involve setting up a server to provide the files?

Answer 1

We ended up setting up nginx to provide files when building. On our build server, the machine building our docker images and the server that has the installer files have a very good connection between them, so downloading huge files is not a real problem.
When it comes to --squash, it is bugged for Docker on Windows. Here is the relevant issue for it:
https://github.com/moby/moby/issues/31468
There is an issue to move --squash out of experimental, but it doesn't seem to have a lot of support:
https://github.com/moby/moby/issues/38657
The alternative that some people propose instead of --squash is multi stage build, discussion here:
https://github.com/moby/moby/issues/34565
There is an alternative to --squash, if you have local installer files, you don't want to set up a web server, and you would like your docker image to be small, and you are running Windows: Use mapped drives.
In Windows, you can share folders with other users on your network. Docker containers are like another computer that is running on your physical machine, and it can access these network drives.
First set up a new user, for example username share and password password1. Create a folder somewhere on your computer. Then right click it, click properties, and then go to the Sharing tab and click "Share". Find the user that you have just created, using the little dropdown menu and Find people ..., and share the folder with this user.
Create a folder somewhere for your test project. Create a batch file setupshare.bat that looks like this:
#echo off
for /f "tokens=2 delims=:" %%i in ('ipconfig ^| findstr "Default Gateway"') do (
set hostip=%%i
goto :end
)
:end
set hostip=%hostip: =%
net use O: \\%hostip%\vms /USER:share password1
The first part of this file is only to find the ip address that the docker container can use to access its host computer. It is not the most pretty thing I've ever put together, so let me know if there's a better way!
It uses a for-loop, as that is the way to save the output of a command to a variable in batch files. The command is ipconfig, and we pipe it to findstr and searches for Default Gateway. We need to use ^| instead of just | because it is in a for-loop. The first part of the for-loop divides each line from the command on the delimiter, which is : in this case, and we only take the second token. The for-loop only handles the first line, if there are multiple entries with a Default Gateway. This script doesn't work if there are multiple entries and the first one is not the correct one.
The line set hostip=%hostip: =% is to remove a space at the start of the string.
We then have the IP address that we want to use stored in hostip. We use this in the net use command, which will map O:\ to shared folder vms on the machine with IP hostip. We use the username share and the password password1. Note that this is a very bad way of handling passwords, as they kind of should be secret!
With a batch file like this, we can set up a Dockerfile in this way:
# escape=`
FROM mcr.microsoft.com/dotnet/core/sdk:3.0
COPY setupshare.bat .
RUN setupshare.bat && `
copy O:\file.txt file.txt
The RUN command will first call setupshare.bat that sets up the network share properly. We can then use any file that we shared, for example a huge installer, and install the things that we want. In this case I have only shared a test file file.txt to see that it works, so just change that line.
I would still advice everyone to just set up a little web server, for example nginx, and use the standard way of writing Dockerfiles, with downloading files and running it in the same RUN command. That's what people expect when they see a Dockerfile, and it should be a more robust solution.
We can also hope that the Docker people either makes a COPY command that can copy, run, and delete installers in the same layer, or that --squash is implemented properly.