* Dockerfile broken with old alpine version with old go
3.4 no longer works with error
```
package context: unrecognized import path "context" (import path does not begin with hostname)
```
3.5 with no changes did not work with error
```
# runtime/cgo
/tmp/go-build671992352/runtime/cgo/_obj/_cgo_export.c:2:20: fatal error: stdlib.h: No such file or directory
#include <stdlib.h>
^
compilation terminated.
```
3.6 with no changes did not work with error:
```
# github.com/xenolf/lego
/usr/lib/go/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find Scrt1.o: No such file or directory
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find crti.o: No such file or directory
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find -lpthread
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find -lssp_nonshared
collect2: error: ld returned 1 exit status
```
* Fix git tag to freeze successfully builded image.
Fix git tag to freeze successfully builded image (prevents issues with newer versions in the future, plus uses stable release)
* Update Dockerfile according to PR comments
* Forgot /go in rm
* Bump lego version
I have modified the official Dockerfile and made two changes
- Each RUN instruction creates an additional layer in the docker image. Adding files in one RUN instruction and deleting it in another RUN instruction will not reduce the size of the image. I used a single RUN command so that all the package/file removal happens in the same command in which the package installation happens, so that no additional layers are created.
- Similar to RUN statement, ADD instruction also creates an additional layer. Using git clone in the RUN statement and deleting it within the same statement ensures that an additional layer with source files is not added.
