[Enhancement]: Resolve variables (ARGs) during Docker image build

[HofmeisterAn]

Problem

Testcontainers for .NET cannot resolve variables (ARGs) during the Docker image build process. Currently, Testcontainers does not pull base images in advance that utilize variables. Hence, if the images are not already present on the host, they will not be pulled, causing the Docker image build process to fail.

• Relates [Bug]: ImageFromDockerfileBuilder cannot build Dockerfile with intermediate layer (regression works with 3.3.0) #972

Solution

To pull base images that utilize variables in advance too, Testcontainers needs to have the capability to resolve the default values and build arguments of variables.

Benefit

Developers will not be required to manually pull the images beforehand, enabling a more streamlined Docker image building and testing configuration.

Alternatives

Pull images manually beforehand.

Would you like to help contributing this enhancement?

Yes

[HofmeisterAn]

Resolving variables (ARGs) should be done here (the matched lines contains already an arg Regex group):

testcontainers-dotnet/src/Testcontainers/Images/DockerfileArchive.cs

Lines 69 to 115 in 604ae2c

	/// <summary>
	/// Gets a collection of base images.
	/// </summary>
	/// <remarks>
	/// This method reads the Dockerfile and collects a list of base images. It
	/// excludes stages that do not correspond to base images. For example, it will not include
	/// the second line from the following Dockerfile configuration:
	/// <code>
	/// FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
	/// FROM build
	/// </code>
	/// </remarks>
	/// <returns>An <see cref="IEnumerable{T}" /> of <see cref="IImage" />.</returns>
	public IEnumerable<IImage> GetBaseImages()
	{
	const string imageGroup = "image";
	var lines = File.ReadAllLines(Path.Combine(_dockerfileDirectory.FullName, _dockerfile.ToString()))
	.Select(line => line.Trim())
	.Where(line => !string.IsNullOrEmpty(line))
	.Where(line => !line.StartsWith("#", StringComparison.Ordinal))
	.Select(line => FromLinePattern.Match(line))
	.Where(match => match.Success)
	// Until now, we are unable to resolve variables within Dockerfiles. Ignore base
	// images that utilize variables. Expect them to exist on the host.
	.Where(match => !match.Groups[imageGroup].Value.Contains('$'))
	.Where(match => !match.Groups[imageGroup].Value.Any(char.IsUpper))
	.ToArray();
	var stages = lines
	.Select(line => line.Value)
	.Select(line => line.Split(new [] { " AS ", " As ", " aS ", " as " }, StringSplitOptions.RemoveEmptyEntries))
	.Where(substrings => substrings.Length > 1)
	.Select(substrings => substrings[substrings.Length - 1])
	.Distinct()
	.ToArray();
	var images = lines
	.Select(match => match.Groups[imageGroup])
	.Select(group => group.Value)
	.Where(value => !stages.Contains(value))
	.Distinct()
	.Select(value => new DockerImage(value))
	.ToArray();
	return images;
	}

[samba2]

I am missing the parsing of "ARG" as well. I use Testcontainers to smoke test a rather complicated Dockerfile which is configured by builds-args/ ARGS. At the moment I manually maintain a second Dockerfile just for the Testcontainers E2E test.