dockerfile: additional expansions

[hdonnay]

Saw some errors on quayio's instance related to variable expansions.
Checking the buildkit dockerfile frontend, it supports the whole compliment of POSIX expansions and bash-style replace (${var/pat/word/}).

This PR implements all the POSIX expansions and reports a dedicated error for the slash pattern replacement.

---

• Check changes after finding an exemplar problematic Dockerfile

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 56.30%. Comparing base (e8f9aff) to head (29d187a).

Files	Patch %	Lines
rhel/dockerfile/vars.go	85.10%	19 Missing and 2 partials ⚠️
rhel/dockerfile/dockerfile.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1241      +/-   ##
==========================================
+ Coverage   55.76%   56.30%   +0.53%     
==========================================
  Files         265      265              
  Lines       16562    16675     +113     
==========================================
+ Hits         9236     9389     +153     
+ Misses       6365     6323      -42     
- Partials      961      963       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hdonnay]

I've run this through the fuzzer for a while, with no crashers being reported.

fuzz: elapsed: 30m1s, execs: 121110401 (0/sec), new interesting: 51 (total: 388)