Add Dockerfiles for building curl-impersonate on Alpine Linux.
The Dockerfile template file was modified to support Alpine Linux,
mainly by changing the dependency installation from 'apt' to 'apk'.
The resulting alpine images are small (~11mb) and will be uploaded to
Docker hub.
Since the firefox and chrome builds are similar except for the TLS
library used, it makes sense to unify their Dockerfiles. This commit
introduces a template Dockerfile from which both the build Dockerfiles
are generated using the simple 'mustache' template system.
Add layer caching to speed up the CI builds. In addition, optimize the
Dockerfiles by moving all the dependency installations (which are
unlikely to change much) to the initial layers.
In preparation for merging the support for Chrome impersonation, move
all build files to the 'firefox' folder. The two builds will live
separately as they are rather different (using two different SSL
libraries for instance).