Let's try to slim the image (public: 1.1GB -> 734MB)

[RaphaelIT7]

Did funnies, all tests seem to pass (GLuaTest & gmod_tests) / no new issues seem to appear
Could probably try to push it further by unpacking the vpk files and merging all into one content directory to avoid duplicate files (iirc VPK files love to have duplicate content)

Depends on #108 as without the build still fails due to steamcmd

[sarahsturgeon]

I worry about removing content packs - makes everything in them unable to be used for tests

[RaphaelIT7]

Docker slim already removes a lot xd
all content_hl2_*, content_cstrike_*, hl2_textures_*, hl2_sound_vo_english_*,  hl2_sound_misc_000 &  hl2_sound_misc_002 are all already being nuked soo just finishing that a bit for an benefit doens't seem like it would hurt much at that point

Most content either seems to be in hl2_misc_00x.vpk or garrysmod_00x.vpk
The content pack content_hl2_005 &  hl2_sound_misc_001 don't seem to contain any content that is used by any tests / I saw no errors related to missing things

Also removing all the _dir.vpk's of dead packs like content_cstrike_dir.vpk and such solves all the NULL FileHandle errors, as it won't try to read imaginary vpk files anymore