Using 'RUN chmod +x' increases container image size

September 5, 2024

You’ll only ever notice this if you have files that are several if not hundreds of megabytes or if you have relatively small images to begin with, but running RUN chmod +x on a copied file will increase the size of the container image. I’ll demonstrate using Astral’s ‘uv’:

FROM alpine:latest

COPY uv uv

RUN chmod +x uv

Building this image will result in the following:

$ docker images
REPOSITORY         TAG       IMAGE ID       CREATED          SIZE
uv                 large     c530abdb5937   16 minutes ago   53.9MB

On first glance there’s nothing inherently off here, but I’ll try and do better:

@@ -1,5 +1,3 @@
 FROM alpine:latest

-COPY uv uv
-
-RUN chmod +x uv
+COPY --chmod=755 uv uv

After using the ‘—chmod’ option (and there’s also a --chown option) the size decreases noticeably:

$ docker images
REPOSITORY         TAG       IMAGE ID       CREATED             SIZE
uv                 mid       2b6aa72b5770   55 minutes ago      31.4MB
uv                 large     c530abdb5937   55 minutes ago      53.9MB

All this has to do with layers and how the file metadata isn’t taken into account when calculating the delta between layers¹.

It might not make sense to go and change every Dockerfile under your control, but if you’re one of the people who regularly deals with larger files² then it might make sense to at least consider this (and the --chown option).