Enable `CMAKE_COMPILE_WARNING_AS_ERROR` in the `Dockerfile`

[jviotti]

We get various (minor) warnings when building the Dockerfile:

#19 106.4 /source/src/generator/resolver.cc: In lambda function:
#19 106.4 /source/src/generator/resolver.cc:169:39: warning: declaration of 'schema' shadows a previous local [-Wshadow]
#19 106.4   169 |               sourcemeta::core::JSON &schema, const sourcemeta::core::URI &base,
#19 106.4       |               ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
#19 106.4 /source/src/generator/resolver.cc:129:14: note: shadowed declaration is here
#19 106.4   129 |   const auto schema{internal_schema_reader(canonical)};
#19 106.4       |              ^~~~~~
#19 107.7 At top level:
#19 107.7 cc1plus: note: unrecognized command-line option '-Wno-dangling-reference' may have been intended to silence earlier diagnostics
#19 108.4 [ 99%] Linking CXX static library libsourcemeta_registry_generator.a
#19 108.5 [ 99%] Built target sourcemeta_registry_generator
#19 108.5 [ 99%] Building CXX object src/index/CMakeFiles/sourcemeta_registry_index.dir/index.cc.o
#19 108.5 [ 99%] Building CXX object test/unit/generator/CMakeFiles/sourcemeta_registry_generator_unit.dir/generator_resolver_test.cc.o
#19 110.0 /source/test/unit/generator/generator_resolver_test.cc: In member function 'virtual void Generator_Resolver_idempotent_Test::TestBody()':
#19 110.0 /source/test/unit/generator/generator_resolver_test.cc:8:33: warning: unnecessary parentheses in declaration of 'resolver' [-Wparentheses]
#19 110.0     8 |   sourcemeta::registry::Resolver(name);                                        \
#19 110.0       |                                 ^~~~~~
#19 110.0 /source/test/unit/generator/generator_resolver_test.cc:41:3: note: in expansion of macro 'RESOLVER_INIT'
#19 110.0    41 |   RESOLVER_INIT(resolver);
#19 110.0       |   ^~~~~~~~~~~~~
#19 110.0 /source/test/unit/generator/generator_resolver_test.cc:8:33: note: remove parentheses
#19 110.0     8 |   sourcemeta::registry::Resolver(name);                                        \
#19 110.0       |                                 ^~~~~~
#19 110.0 /source/test/unit/generator/generator_resolver_test.cc:41:3: note: in expansion of macro 'RESOLVER_INIT'
#19 110.0    41 |   RESOLVER_INIT(resolver);
#19 110.0       |   ^~~~~~~~~~~~~
#19 110.0 /source/test/unit/generator/generator_resolver_test.cc:33:30: warning: unnecessary parentheses in declaration of 'collection' [-Wparentheses]
#19 110.0    33 |     RESOLVER_COLLECTION_INIT((collection), (collection_name));                 \
#19 110.0       |                              ^~~~~~~~~~~~

i.e. see https://github.com/sourcemeta/registry/actions/runs/15323996661/job/43113883224?pr=182