chore: Apply Docker build check corrections

[matthewfeickert]

Description

• Apply revisions to Dockerfile from Docker build checks.
  • https://docs.docker.com/reference/build-checks/from-as-casing/
  • https://docs.docker.com/go/dockerfile/rule/undefined-var/
  • https://docs.docker.com/go/dockerfile/rule/legacy-key-value-format/

Checklist Before Requesting Reviewer

• [x] Tests are passing
• [x] "WIP" removed from the title of the pull request
• [x] Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• [x] Summarize commit messages into a comprehensive review of the PR

* Apply revisions to Dockerfile from Docker build checks.
   - https://docs.docker.com/reference/build-checks/from-as-casing/
   - https://docs.docker.com/go/dockerfile/rule/undefined-var/
   - https://docs.docker.com/go/dockerfile/rule/legacy-key-value-format/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.21%. Comparing base (609af88) to head (7f4b2b0). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2543 +/- ## ======================================= Coverage 98.21% 98.21% ======================================= Files 69 69 Lines 4543 4543 Branches 804 804 ======================================= Hits 4462 4462 Misses 48 48 Partials 33 33 ``` | [Flag](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | Coverage Δ | | |---|---|---| | [contrib](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `97.79% <ø> (ø)` | | | [doctest](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `98.08% <ø> (ø)` | | | [unittests-3.10](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.23% <ø> (ø)` | | | [unittests-3.11](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.23% <ø> (ø)` | | | [unittests-3.12](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.23% <ø> (ø)` | | | [unittests-3.8](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.25% <ø> (ø)` | | | [unittests-3.9](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2543/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.27% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep#carryforward-flags-in-the-pull-request-comment) to find out more.

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[matthewfeickert]

@meeseeksdev backport to release/v0.7.x