Memory Leak in warmer

[mxbossard]

Actual behavior
When we warm an image it's fully loaded in RAM before it's written on disk.
The TarWriter is a bytes.Buffer here:

kaniko/pkg/cache/warm.go

Line 68 in 8c09eff

TarWriter: tarBuf,

Then the files are written here:

kaniko/pkg/cache/warm.go

Line 84 in 8c09eff

if err := writeBufsToFile(cachePath, tarBuf, manifestBuf); err != nil {

Expected behavior
The downloaded file should be streamed into a file on disk with a FileWritter.

To Reproduce
warmer -i

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing	- [No]

[aaron-prindle]

Thank you for flagging this @mxbossard! Would you want to drive a PR to fix this issue - changing the warmer to stream files vs loading them all into RAM first?

[mxbossard]

I gave it a try and proposed a PR.
I added a shell script in scripts dir to test the warmer in a docker container within boxed memory conditions.
I could reproduce the oom error before the fix and validate the fix.