fix(docker): install custom packages into runtime venv

[KimSeoYeon23]

Summary

This installs custom Python packages into Sentry's runtime virtualenv instead of the
system Python.

self-hosted runs Sentry from /.venv, but sentry/Dockerfile was installing the
bundled nodestore extension with pip, which targets the system interpreter. That can
leave custom packages unavailable at runtime and cause the web service to fail during
startup.

Root cause

The image build used:

RUN pip install ...

while the actual Sentry runtime uses /.venv.

In my case, this caused the nodestore backend package to be installed outside the runtime
environment, which led to repeated web worker exits and an unhealthy web container.

Fix

Use /.venv/bin/pip for custom package installation in sentry/Dockerfile.

This updates:

• the nodestore extension install step
• the optional requirements.txt install step

Validation

After rebuilding the image and restarting the stack:

• the custom package is installed into /.venv
• web workers stop exiting during startup
• sentry-self-hosted-web-1 becomes healthy

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of
Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in
order to utilize my contributions in this here PR. So here's the deal: I retain all
rights, title and interest in and to my contributions, and by keeping this boilerplate
intact I confirm that Sentry can use, modify, copy, and redistribute my contributions,
under Sentry's choice of terms.zhem

[aldy505]

Looks good to me, let's see if the integration tests passed, and I'll merge

[aminvakil]

I do not think this should be merged.

https://github.com/getsentry/sentry/blob/3fb85962b4d7295920f27d687500089cb49beba2/self-hosted/Dockerfile#L42-L50

upstream sentry Dockerfile uses /.venv/bin/ right now and sentry/Dockerfile in this repository already adapts that by executing pip as it is.

Although this does not harm right now, but we should keep up with pip in upstream sentry Dockerfile if we change this.

Right now there is no problem to be solved AFAICS.