use alpine as base image

[pablochacin]

Change the base image to reduce the attack surface and minimize the exposure.

Fixes: #77

Results of image scanning with trivy (after update in #85):

xk6 (debian 12.4)
=================
Total: 360 (UNKNOWN: 3, LOW: 247, MEDIUM: 75, HIGH: 34, CRITICAL: 1)

alpine-based image:

xk6:alpine-3.18 (alpine 3.18.6)
===============================
Total: 0 (UNKNOWN: 0, LOW: 0, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

Note: There's a newer version of alpine based image (3.19.1) but trivy issues a warning when scanning it:

2024-01-29T11:55:10.355Z        WARN    This OS version is not on the EOL list: alpine 3.19

[szkiba]

LGTM

[pablochacin]

Reverted in #87 due to concerns in breaking changes due to alpine not using standard clib and lack of some tools that may be required by images that use xk6 image as a base.