TL;DR The contents of directories mounted with
--mount=type=cache
are not stored in the docker image, so it makes sense to cache intermediate directories, rather than target ones.
In dockerfile:1.3
there is a feature of mounting file system directories during the build process, that can be used for caching downloaded packages or compilation artifacts.
For example, the uwsgi
package must be compiled every time it is installed, and at first glance, build times can be reduced by making the entire Python package directory cacheable:
# syntax=docker/dockerfile:1.3
FROM python:3.10
RUN mkdir /pip-packages
RUN --mount=type=cache,target=/pip-packages \
pip install --target=/pip-packages uwsgi
> docker build -t pip-cache -f Dockerfile.pip .
# ...
[+] Building 14.6s (7/7) FINISHED
Looks like everything went well, but the target directory is empty:
> docker run -it --rm pip-cache ls -l /pip-packages
total 0
Something is definitely wrong. You can see that during the build uWSGI was compiled and installed. You can even check it adding ls
in the build process:
RUN --mount=type=cache,target=/pip-packages \
pip install --target=/pip-packages uwsgi \
&& ls -1 /pip-packages
> docker build -t pip-cache --progress=plain -f Dockerfile.pip .
<...>
#6 12.48 Successfully installed uwsgi-2.0.20
<...>
#6 12.91 __pycache__
#6 12.91 bin
#6 12.91 uWSGI-2.0.20.dist-info
#6 12.91 uwsgidecorators.py
#6 DONE 13.0s
<...>
Everything is in its place. But the final image is empty again:
> docker run -it --rm pip-cache ls -l /pip-packages
total 0
The thing is, the /pip-packages
catalog, that is inside the image, and the catalog, that is in RUN --mount=type=cache,target=<dirname>
, are completely different. Let's try to put something inside this directory and track its contents during the build process:
RUN mkdir /pip-packages \
&& touch /pip-packages/foo \
&& ls -1 /pip-packages
RUN --mount=type=cache,target=/pip-packages \
ls -1 /pip-packages \
&& pip install --target=/pip-packages uwsgi \
&& ls -1 /pip-packages
RUN ls -1 /pip-packages
> docker build -t pip-cache --progress=plain -f Dockerfile.pip-track .
<...>
#5 [stage-0 2/4] RUN mkdir /pip-packages
&& touch /pip-packages/foo
&& ls -1 /pip-packages
#5 sha256:fb542<...>
#5 0.211 foo 👈1️⃣
#5 DONE 0.2s
#6 [stage-0 3/4] RUN --mount=type=cache,target=/pip-packages
ls -1 /pip-packages
&& pip install --target=/pip-packages uwsgi
&& ls -1 /pip-packages
#6 sha256:10ed6<...>
#6 0.292 __pycache__ 👈2️⃣
#6 0.292 bin
#6 0.292 uWSGI-2.0.20.dist-info
#6 0.292 uwsgidecorators.py
#6 2.802 Collecting uwsgi 🤔3️⃣
#6 3.189 Downloading uwsgi-2.0.20.tar.gz (804 kB)
#6 4.400 Building wheels for collected packages: uwsgi
<...>
#6 13.34 __pycache__ 👈4️⃣
#6 13.34 bin
#6 13.34 uWSGI-2.0.20.dist-info
#6 13.34 uwsgidecorators.py
#6 DONE 13.4s
#7 [stage-0 4/4] RUN ls -1 /pip-packages
#7 sha256:fb6f4<...>
#7 0.227 foo 👈5️⃣
#7 DONE 0.2s
<...>
- 1️⃣ file
foo
created successfully - 2️⃣ the directory with the results of the previous
docker build
was mounted, and there's nofoo
file - 3️⃣ uWSGI is downloaded, compiled and installed again
- 4️⃣ an updated uWSGI package appeared in the catalog
- 5️⃣ only the file
foo
is left in the directory
This means that --mount=type=cache
only works in the context of a single RUN
instruction, replacing the directory created inside the image with RUN mkdir /pip-packages
and then reverting it back. Also, caching turned out to be ineffective because pip
reinstalled uWSGI with a full compilation.
In this case, it would be correct to cache not the target directory, but /root/.cache
, where pip
stores all the artifacts:
RUN --mount=type=cache,target=/root/.cache \
pip install --target=/pip-packages uwsgi
> docker build -t pip-cache -f Dockerfile.pip-right .
> docker run -it --rm pip-cache ls -1 /pip-packages
__pycache__
bin
uWSGI-2.0.20.dist-info
uwsgidecorators.py
Now everything is in place, the installed packages have not disappeared.
Let's check the effectiveness of caching by adding the requests
package:
RUN --mount=type=cache,target=/root/.cache \
pip install --target=/pip-packages uwsgi requests
👆
> docker build -t pip-cache --progress=plain -f Dockerfile.pip-right .
<...>
#6 6.297 Collecting uwsgi
#6 6.297 Using cached uWSGI-<...>.whl 👈
#6 6.561 Collecting requests
#6 6.980 Downloading requests-2.27.1-py2.py3-none-any.whl (63 kB)
<...>
pip
used the pre-built wheel file from /root/.cache
and installed a ready-to-use package from it.
All sources are available on GitHub.
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