Fixes #676: Update dockerfile; Update alpine base images; Fix multiple CVEs

[hilariocoelho]

Fixes #676 by updating alpine base image.

Current image:

% syft appleboy/gorush --output json | grype                                                                     
 ✔ Loaded image            
 ✔ Parsed image            
 ✔ Cataloged packages      [102 packages]

[0001]  WARN some package(s) are missing CPEs. This may result in missing vulnerabilities. You may autogenerate these using: --add-cpes-if-none
NAME                                 INSTALLED  FIXED-IN   TYPE       VULNERABILITY        SEVERITY 
busybox                              1.34.1-r3  1.34.1-r5  apk        CVE-2022-28391       Critical  
github.com/google/flatbuffers        v1.12.0               go-module  CVE-2020-35864       High      
github.com/prometheus/client_golang  v1.10.0               go-module  CVE-2022-21698       High      
github.com/tidwall/gjson             v1.6.8     1.9.3      go-module  GHSA-ppj4-34rq-v8j9  High      
google.golang.org/protobuf           v1.28.0               go-module  CVE-2015-5237        High      
google.golang.org/protobuf           v1.28.0               go-module  CVE-2021-22570       High      
libcrypto1.1                         1.1.1l-r7  1.1.1n-r0  apk        CVE-2022-0778        High      
libcrypto1.1                         1.1.1l-r7             apk        CVE-2021-4160        Medium    
libretls                             3.3.4-r2   3.3.4-r3   apk        CVE-2022-0778        High      
libssl1.1                            1.1.1l-r7             apk        CVE-2021-4160        Medium    
libssl1.1                            1.1.1l-r7  1.1.1n-r0  apk        CVE-2022-0778        High      
ssl_client                           1.34.1-r3  1.34.1-r5  apk        CVE-2022-28391       Critical  
zlib                                 1.2.11-r3  1.2.12-r0  apk        CVE-2018-25032       High  

After alpine image update:

% syft spawn2kill/gorush --output json | grype
 ✔ Loaded image            
 ✔ Parsed image            
 ✔ Cataloged packages      [102 packages]
[0001]  WARN some package(s) are missing CPEs. This may result in missing vulnerabilities. You may autogenerate these using: --add-cpes-if-none
NAME                           INSTALLED  FIXED-IN  TYPE       VULNERABILITY        SEVERITY 
github.com/google/flatbuffers  v1.12.0              go-module  CVE-2020-35864       High      
github.com/tidwall/gjson       v1.6.8     1.9.3     go-module  GHSA-ppj4-34rq-v8j9  High      
google.golang.org/protobuf     v1.28.0              go-module  CVE-2021-22570       High      
google.golang.org/protobuf     v1.28.0              go-module  CVE-2015-5237        High      

[appleboy]

https://github.com/appleboy/gorush/releases/tag/v1.16.0