shrinking a Docker Image - from debian to scratch - how to migrate? - dockerfile

Question

i am trying to build a minimalistic docker image for one of my applicatoins
in my "usual" builds i do not rely on 3rd party applications. This time I need to include a precompiled executeable (xpdf) to the build; My go applications are prebuilt in a builder Docker and then copied over (no dependencies).
my current Dockerimage file looks like this: (working!) application launches
FROM debian:buster-slim
COPY ./bin64/pdftopng .
COPY ./config/xpdfrc .
# ... copy FONT files
# copy Test PDF
COPY ./test.pdf .
RUN chmod +x pdftopng
RUN apt-get update && apt-get install -y libfontconfig1
# execute pdftopng
ENTRYPOINT ["/pdftopng", "-cfg", "xpdfrc", "/test.pdf", "/test.pdf"]
as this results in a image file with over 100mb i tried to use scratch as a base (which usually results in images around 10-20mb)
FROM scratch
# ... copy my binaries to the docker
COPY --from=builder /dist/main /
# copy the XPDF -> pdf to png executeable (linux x64)
COPY ./bin64/pdftopng .
# copy font files (for this example i only copy one)
COPY ./fonts/n021003l.pfb /fonts/n021003l.pfb
# copy config file
COPY ./config/xpdfrc .
# copy test file
COPY ./test.pdf .
# execute the XPDF converter and convert the test.pdf to png
ENTRYPOINT ["/pdftopng", "-cfg", "xpdfrc", "/test.pdf", "test"]
outputs:
standard_init_linux.go:219: exec user process caused: no such file or directory
this error is the result of a missing library (something related to libfontconfig1)
how can i find out which files are missing (as i do have a working container this should be possible)

Answer 1

Solution
Step 1 - create libsource
create a docker image where you can grab all required libraries from
FROM debian:buster-slim AS libsource
COPY ./bin64/pdftopng .
COPY ./config/xpdfrc .
# ... copy FONT files
# copy Test PDF
COPY ./test.pdf .
RUN chmod +x pdftopng
RUN apt-get update && apt-get install -y libfontconfig1
Step 2 - find requirements
then obtain a list of libraries which the application [./pdftopng] needs to run
docker run --rm -it ldd ./pdftopng
this command lists all required libraries (direct use only):
linux-vdso.so.1 (0x00007ffce1304000)
libbz2.so.1 => /lib/x86_64-linux-gnu/libbz2.so.1 (0x00007fa1104ea000)
libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fa1102cc000)
libfontconfig.so.1 => /usr/lib/x86_64-linux-gnu/libfontconfig.so.1 (0x00007fa110286000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa110265000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fa1100e1000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fa10ff5e000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fa10ff42000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa10fd81000)
libfreetype.so.6 => /usr/lib/x86_64-linux-gnu/libfreetype.so.6 (0x00007fa10fcc5000)
libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007fa10fc88000)
libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007fa10fc7f000)
/lib64/ld-linux-x86-64.so.2 (0x00007fa110501000)
libpng16.so.16 => /usr/lib/x86_64-linux-gnu/libpng16.so.16 (0x00007fa10fc44000)
Step 3 - copy requirements
in the last step copy the libraries from step 2 to the scratch image
COPY --from=libsource /lib/x86_64-linux-gnu/libbz2.so.1 /lib/x86_64-linux-gnu/libbz2.so.1
COPY --from=libsource /lib/x86_64-linux-gnu/libz.so.1 /lib/x86_64-linux-gnu/libz.so.1
COPY --from=libsource /usr/lib/x86_64-linux-gnu/libfontconfig.so.1 /usr/lib/x86_64-linux-gnu/libfontconfig.so.1
COPY --from=libsource /lib/x86_64-linux-gnu/libpthread.so.0 /lib/x86_64-linux-gnu/libpthread.so.0
COPY --from=libsource /usr/lib/x86_64-linux-gnu/libstdc++.so.6 /usr/lib/x86_64-linux-gnu/libstdc++.so.6
COPY --from=libsource /lib/x86_64-linux-gnu/libm.so.6 /lib/x86_64-linux-gnu/libm.so.6
COPY --from=libsource /lib/x86_64-linux-gnu/libgcc_s.so.1 /lib/x86_64-linux-gnu/libgcc_s.so.1
COPY --from=libsource /lib/x86_64-linux-gnu/libc.so.6 /lib/x86_64-linux-gnu/libc.so.6
COPY --from=libsource /usr/lib/x86_64-linux-gnu/libfreetype.so.6 /usr/lib/x86_64-linux-gnu/libfreetype.so.6
COPY --from=libsource /lib/x86_64-linux-gnu/libexpat.so.1 /lib/x86_64-linux-gnu/libexpat.so.1
COPY --from=libsource /lib/x86_64-linux-gnu/libuuid.so.1 /lib/x86_64-linux-gnu/libuuid.so.1
COPY --from=libsource /lib64/ld-linux-x86-64.so.2 /lib64/ld-linux-x86-64.so.2
COPY --from=libsource /usr/lib/x86_64-linux-gnu/libpng16.so.16 /usr/lib/x86_64-linux-gnu/libpng16.so.16
Result
in my case the result is a much smaller image 15.45 MB instead of 101.90 MB
which is a reduction by 84,84 %