Make gitrest docker build install with a frozen lockfile

[Abe27342]

Description

Main desirable change: updates the gitrest docker build to copy .npmrc within the docker container before installing, which ensures the subsequent install includes frozen-lockfile=true.

Fallout from this desire: the previous CI setup for docker builds involved using flub to set the package version before the docker build. This makes pnpm install with frozen lockfile fail, since the dependency versions of packages within the workspace will be whatever that package version was set to, rather than the version within source control. To fix this, I've updated the docker build to support setting the package version from within the docker container using flub, and plumbed through necessary variables in CI to make that possible.

Also includes some minor quality improvements to the docker build, such as fixing up deprecated syntax warnings, using a cache mount for the pnpm dependencies, and reducing the footprint of the runner build slightly (there's still much more that could be done in that area if we care to invest: we keep around dev deps, for example).

AB#26779

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[Abe27342]

Sample build here:

https://dev.azure.com/fluidframework/internal/_build/results?buildId=316564&view=results

Plan is to transition other server pipelines to a similar structure, but I want a bit more e2e verification first.

[alexvy86]

LGTM

[Abe27342]

ok, finally got e2e tests working. looks healthy--https://dev.azure.com/fluidframework/internal/_build/results?buildId=316917&view=results (note the LTS build is from a previous attempt that didn't get requeued as I didn't bother, therefore pass rate much lower than usual)