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Docker Intermediate: Images and Compose That Survive Real Projects
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Docker Intermediate: Images and Compose That Survive Real Projects

Check Yourself: Data That Outlives Containers

Five scenario questions on mounts, copy-in behaviour, speed, ownership and read-only errors. If you miss more than one, reread the lessons in this section.

5 min DockerSecurityDeployment and ContainersPackaging and Dependencies Intermediate
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1. You mount an empty folder from your Windows disk over /usr/share/nginx/html and nginx now answers 404 for every page. Which statement explains it?
Bind mounts only work with databases. A bind mount shows the folder you give it and hides what the image had at that path, so the container sees an empty folder. nginx cannot read folders from Windows. Docker deleted the files in the image when you mounted the folder.
2. You mount a brand-new named volume at a path where the image already has files. What do you see in the container?
Only the files that are newer than the volume. An empty folder, the same as a bind mount. An error saying the path already exists. The image's files, because a new empty named volume is filled from the image on first use.
3. A Postgres container on Windows is slow with a bind-mounted folder and fast with a named volume, using the same image. What is the most likely reason?
Bind mounts disable the database's cache. Windows throttles containers that use bind mounts. Named volumes use a faster version of Postgres. A bind mount of a Windows folder crosses from Linux into the Windows file system for every file operation, while a named volume stays on the Linux side.
4. A non-root container gets Permission denied when it writes to a freshly created named volume. docker run with stat shows the folder owned by root with mode drwxr-xr-x. What is a clean fix for your own image?
In the Dockerfile, create the folder and give it to the app user before the volume is first used, so a new volume inherits that owner. Run the container as root forever. Delete the image and pull it again. Use chmod 777 on the folder.
5. A container fails with the message Read-only file system when it writes to a mounted folder, and with Permission denied when it writes to a different mounted folder. What does that tell you?
Docker is out of memory in both cases. The first folder is full and the second is missing. The first mount was written with :ro, and the second one has an owner or mode that does not allow this user. Both messages mean the same thing, and a chmod fixes both.

About

Five scenario questions on mounts, copy-in behaviour, speed, ownership and read-only errors. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
08/01/2026
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