Simplify from scratch docker - use built in go CA bundle

[ldemailly]

Fixes #145

• adding local docker test: shows:

  18:21:00 [E] 2: Error fetching 192.9.142.5: Get "https://debug.fortio.org/build-test": tls: failed to verify certificate: x509: 
  certificate signed by unknown authority
  

  when missing CA bundle
• remove the certs alpine docker step and test should fail
• replace it by _ "golang.org/x/crypto/x509roots/fallback" import
• move that to cli so it applies to all

[ldemailly]

negative test to test the build tag from fortio/cli#31

$ make no-bundle-failing-test
CGO_ENABLED=0 GOOS=linux go build -a -tags no_tls_fallback .
docker build -t fortio/multicurl:local -f Dockerfile .
[+] Building 0.1s (5/5) FINISHED                                                                                                                                                      docker:desktop-linux
 => [internal] load build definition from Dockerfile                                                                                                                                                  0.0s
 => => transferring dockerfile: 118B                                                                                                                                                                  0.0s
 => [internal] load .dockerignore                                                                                                                                                                     0.0s
 => => transferring context: 2B                                                                                                                                                                       0.0s
 => [internal] load build context                                                                                                                                                                     0.1s
 => => transferring context: 7.46MB                                                                                                                                                                   0.1s
 => [1/1] COPY multicurl /usr/bin/multicurl                                                                                                                                                           0.0s
 => exporting to image                                                                                                                                                                                0.0s
 => => exporting layers                                                                                                                                                                               0.0s
 => => writing image sha256:29cbb54a02b023d0b55dd66c1a05ca8e056b72bb90c7e1b05e9cbe2968540713                                                                                                          0.0s
 => => naming to docker.io/fortio/multicurl:local                                                                                                                                                     0.0s

View build details: docker-desktop://dashboard/build/desktop-linux/desktop-linux/d457fpg80e9q8llpbu1u1qi7a

What's next:
    View a summary of image vulnerabilities and recommendations → docker scout quickview 
docker run --rm fortio/multicurl:local -4 https://debug.fortio.org/build-test
19:19:05 [I] Fortio multicurl dev  go1.22.4 arm64 linux, using resolver ip4, GET https://debug.fortio.org/build-test
19:19:05 [I] Resolved ip4 debug.fortio.org:https to port 443 and 3 addresses [192.9.227.83 18.222.136.83 192.9.142.5]
19:19:05 [E] 1: Error fetching 192.9.227.83: Get "https://debug.fortio.org/build-test": tls: failed to verify certificate: x509: certificate signed by unknown authority
19:19:05 [E] 2: Error fetching 18.222.136.83: Get "https://debug.fortio.org/build-test": tls: failed to verify certificate: x509: certificate signed by unknown authority
19:19:05 [E] 3: Error fetching 192.9.142.5: Get "https://debug.fortio.org/build-test": tls: failed to verify certificate: x509: certificate signed by unknown authority
19:19:05 [E] [1] 3 errors (0 warnings)
19:19:05 [E] Reached max repeat 0
19:19:05 [I] Total iterations: 1, errors: 3, warnings 0
make: *** [test-local-image] Error 3