fix dockerfile + parameterized http host setting

[renatopancheri]

Greetings,

this pull request addresses 2 specific issues i encountered:

1. fix dockerfile: in my environment (ubuntu 24.04 docker.io/noble-updates,now 26.1.3-0ubuntu1~24.04.1) docker build would fail when running
   COPY config.ru saml_proxy.rb LICENSE "$APP_ROOT
   adding a final slash fixed the issue for me

2. i wanted the ability to have multiple applications each one of them with their unique FQDN all with SAML auth with a single instance of nginx and saml-proxy, i was able to do this with this patch and running saml-proxy with:
   SAML_ASSERTION_CONSUMER_SERVICE_URL=https://%HOST%/consume
   and finally i had to add:
   proxy_set_header Host $host;
   on /start and /consume endpoint on nginx

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 31.81818% with 15 lines in your changes missing coverage. Please review.

Project coverage is 82.28%. Comparing base (4030903) to head (86b4f84).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
saml_proxy.rb	31.81%	15 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (4030903) and HEAD (86b4f84). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (4030903)	HEAD (86b4f84)
	4	3

Additional details and impacted files

@@             Coverage Diff              @@
##              main     #257       +/-   ##
============================================
- Coverage   100.00%   82.28%   -17.72%     
============================================
  Files           10       10               
  Lines          236      254       +18     
============================================
- Hits           236      209       -27     
- Misses           0       45       +45     

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