Dockerfile:checksums+gofix+alpine3.8

[moritzraho]

PR grouping all three #71 #73 #74 + test-nginx 0.24 -> 0.26:

• Verify checksums of downloaded files in Dockerfile
• Update to Alpine 3.8 to fix alpine vulnerability
• Gofix for GO error when building Dockerfile #72
• test-nginx 0.24 -> 0.26