Improve the caching behavior of web/Dockerfile.

[briansmith]

Previously if anything changed under web/ then the images would get
completely rebuilt. In particular fetching new packages in
yarn install can be very slow.

Attempt to minimize file dependencies in earlier stages of the Dockerfile
to maximize the number of layers that can be cached, both in the web
asset stage and the go server stage.

Signed-off-by: Brian Smith [email protected]

[briansmith]

@rmars @klingerf There's no rush for this. TBH, this is more of a "does this idea make sense at all" question rather than a serious PR because I haven't even tested the resultant image, beyond verifying that bin/docker-build-web succeeds. In particular, I am assuming yarn install only package.json and yarn.lock to execute correctly based on seeing others do it, e.g. yarnpkg/yarn#749 (comment). Also I'm assuming that the Go building step doesn't require any non-Go source files.

[rmars]

when I run docker-compose build && docker-compose up -d on this branch, then visit the web UI, I get errors:

web_1          | 2018/02/16 21:02:25 http: panic serving 172.21.0.1:58688: runtime error: invalid memory address or nil pointer dereference
web_1          | goroutine 54 [running]:
web_1          | net/http.(*conn).serve.func1(0xc420082780)
web_1          | 	/usr/local/go/src/net/http/server.go:1697 +0xd0
web_1          | panic(0xd43760, 0x139d080)
web_1          | 	/usr/local/go/src/runtime/panic.go:491 +0x283
web_1          | html/template.(*Template).lookupAndEscapeTemplate(0x0, 0xe4bfd1, 0x4, 0x0, 0x0, 0x0)
web_1          | 	/usr/local/go/src/html/template/template.go:144 +0x50
web_1          | html/template.(*Template).ExecuteTemplate(0x0, 0x7faaa811d470, 0xc4204702d0, 0xe4bfd1, 0x4, 0xd80460, 0xc4202fa0e0, 0x7faaa8179ef0, 0xcf6660)
web_1          | 	/usr/local/go/src/html/template/template.go:133 +0x43
web_1          | github.com/runconduit/conduit/web/srv.(*Server).RenderTemplate(0xc42006e3c0, 0x7faaa81803f0, 0xc4204702d0, 0xe548fa, 0xd, 0xe4bfd1, 0x4, 0xdbe680, 0xc420039590, 0xc420039590, ...)
web_1          | 	/go/src/github.com/runconduit/conduit/web/srv/server.go:115 +0x314
web_1          | github.com/runconduit/conduit/web/srv.(*Server).RenderTemplate-fm(0x7faaa81803f0, 0xc4204702d0, 0xe548fa, 0xd, 0xe4bfd1, 0x4, 0xdbe680, 0xc420039590, 0x0, 0x0)
web_1          | 	/go/src/github.com/runconduit/conduit/web/srv/server.go:69 +0x8d
web_1          | github.com/runconduit/conduit/web/srv.(*handler).handleIndex(0xc4200398f0, 0x7faaa81803f0, 0xc4204702d0, 0xc420412900, 0x0, 0x0, 0x0)
web_1          | 	/go/src/github.com/runconduit/conduit/web/srv/handlers.go:35 +0x1d1
web_1          | github.com/runconduit/conduit/web/srv.(*handler).(github.com/runconduit/conduit/web/srv.handleIndex)-fm(0x7faaa81803f0, 0xc4204702d0, 0xc420412900, 0x0, 0x0, 0x0)
web_1          | 	/go/src/github.com/runconduit/conduit/web/srv/server.go:82 +0x66
web_1          | github.com/julienschmidt/httprouter.(*Router).ServeHTTP(0xc4200392c0, 0x7faaa81803f0, 0xc4204702d0, 0xc420412900)
web_1          | 	/go/src/github.com/julienschmidt/httprouter/router.go:299 +0x6f1
web_1          | github.com/runconduit/conduit/web/srv.(*Server).ServeHTTP(0xc42006e3c0, 0x7faaa81803f0, 0xc4204702d0, 0xc420412900)
web_1          | 	/go/src/github.com/runconduit/conduit/web/srv/server.go:49 +0x4d

probably due to the template files not being where the web server expects them

[briansmith]

Thanks @rmars I didn't realize that templates/ has to be copied too. I didn't (don't) quite know how the web parts work.

Anyway, I'm going to drop this. I'm not in a good position to validate changes to the web/ component like this and I've no context on what would actually be optimal. I'll let the branch continue to exist for a while in case somebody cares about this and wants to take it over the line.