Adjust installation order and purge "libpythonXX.a" files

[tianon]

Closes #507
Closes #503
Closes #504

[tianon]

(Cognitively, I think it makes sense to finish everything we need to do with the source code, then purge all traces of the source code, then purge files we didn't want to have installed, then detect necessary dependencies, hence my reordering.)

[tianon]

A couple size comparisons (before / after):

• python:3.8-alpine3.12: 80.3MB / 42.7MB
  • https://github.com/docker-library/python/runs/921941015?check_suite_focus=true#step:8:12
  • https://github.com/docker-library/python/pull/508/checks?check_run_id=934002412#step:8:14
• python:3.8-slim-buster: 165MB / 168MB
  • https://github.com/docker-library/python/runs/921940999?check_suite_focus=true#step:8:15
  • https://github.com/docker-library/python/pull/508/checks?check_run_id=934002405#step:8:15

(That slight increase on 3.8-slim-buster is curious 👀 )

[tianon]

I built before/after locally to help minimize differences in the packages installed, etc to try and narrow down why python:3.8-slim-buster got slightly bigger:

• Before: ~165MB
• After: ~169MB

(In other words, building locally didn't actually help narrow down anything....)

This will have to be a project for next week. 😞

[Lebvanih]

Hello,

The size increase that you see on the debian buster slim seems to come from the ordering.
If I take the new docker file and stop it after the build of python (so before the part for PIP) and then check the amount of package installed on the system:
root@b96d018c9659:/usr# dpkg --list | wc -l
123

If I check the same on the old image:
root@e1e2a2280dd3:/usr# dpkg --list | wc -l
98

I am not really an expert with ldconfig and what it does and how this affect the whole build, but it seems that if ldconfig is placed at the end, less package are cleaned during the apt-get purge resulting in more space used in /usr/lib and /usr/share directory:
After:

root@b96d018c9659:/usr# du -xhs *
14M	bin
4.0K	games
9.8M	include
55M	lib
26M	local
2.0M	sbin
38M	share
4.0K	src

Before:

14M	bin
4.0K	games
4.0K	include
33M	lib
85M	local
2.0M	sbin
6.9M	share
4.0K	src

The line of the dockerfile i'm mentioning:

find /usr/local -type f -executable -not \( -name '*tkinter*' \) -exec ldd '{}' ';' \
		| awk '/=>/ { print $(NF-1) }' \
		| sort -u \
		| xargs -r dpkg-query --search \
		| cut -d: -f1 \
		| sort -u \
		| xargs -r apt-mark manual \
	&& apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false \

I tried to move the ldconfig line above the two mentioned above and I got back to 98 packages visible from dpkg --list.
If I recheck on my image (still without pip for the comparison):

root@2e927fb79cf3:/usr# du -xhs *
14M	bin
4.0K	games
4.0K	include
33M	lib
26M	local
2.0M	sbin
6.9M	share
4.0K	src

Adding the part with pip I get this size:

REPOSITORY                                               TAG                 IMAGE ID            CREATED             SIZE
cleaned3                                                 latest              449a3f524048        5 seconds ago       113MB

The only difference with your dockerfile is the place of the ldconfig.
How it looks on mine:

       && apt-mark auto '.*' > /dev/null \
	&& apt-mark manual $savedAptMark \
	\
	&& ldconfig \
	&& find /usr/local -type f -executable -not \( -name '*tkinter*' \) -exec ldd '{}' ';' \
		| awk '/=>/ { print $(NF-1) }' \
		| sort -u \
		| xargs -r dpkg-query --search \
		| cut -d: -f1 \
		| sort -u \
		| xargs -r apt-mark manual \
	&& apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false \
	&& rm -rf /var/lib/apt/lists/* \
	\
	&& python3 --version

Hoping this helps, as I said, I am not an expert, so I am not sure why this is affecting, but as you moved it in this commit, I tried to see if that could be the issue of the size change.

[tianon]

Yeah, great catch! I realized right after I left for the day on Friday that I can't ldd before I ldconfig, and that was probably exactly the reason for the discrepancy. 🤦

After fixing that:

• python:3.8-slim-buster before: ~172MB (freshly built)
• python:3.8-slim-buster after: ~120MB

Much better 😇

[tianon]

This helped me notice #439 (comment) (we missed slim-buster variants entirely for 3.9-rc, so we should add those back after we finish this).